If the U.S. Constitution was rewritten from scratch to reflect the many changes that have taken place in society since the original document was written, how different would it be from the original Constitution?

This is a common misconception about Constitutional legislation. A constitution is a mission statement for how the government should conduct itself, it should frame the context in which laws are written only. A good constitution should be vague on specifics unless it has to do with defining the power structure or very important values like freedom of speech. The U.S. Constitution does a good job on all fronts in these respects. The issue we have right now is that we have a culture in Washington that refuses to take responsibility for what it has been assigned to do. This is why there are multiple issues that are not being addressed. When you have a very specific constitution with lots of well meaning liberal components it makes it very hard to keep it up to date and increases the likelihood political figures will manipulate the complexity. An example is the constitution of the Wiemar Republic. On paper it is a better constitution than the one here in the U.S. by pretty much every measure of the word "better", but because of its complexity it was easily manipulated by Hitler and the Nazi's. Indeed I think it was well meaning people like the person that wrote this question that drew up the Wiemar Constitution. As for the U.S. Constitution it has few real flaws and the ones it does have can be addressed via the amendment process which is hard right now due to the lack of good governance. But most of the things that change over time should be laws outside of the constitution. Just because something is not in the constitution does not mean it is less important for instance Social Security is not in the constitution. Some things I would like addressed in the Constitution before I die are: Clearer language on the constitutional convention process and having one mandated say every 20 years, which is what was originally intended when it was drafted by many of the participants at the original convention. Term limits for all congressional leaders, justices from the Court of Appeals, and the Supreme Court Popular Vote of the President A better way of selecting VP's than just being hand picked without anyone outside of the nominating party voting on it.

The question asks "how different" instead of "what ways would it be different" so (thankfully for all involved) I won't go into specific details. What isn't clear by the question is whether the original intent of the constitution would stay or if instead this time the will of the people would be represented. One of James Madison's driving objectives in writing the constitution was to make change very difficult and to help ensure the power and prestige of the elite (the landed elite in his day). In this respect I don't think many changes would need to be made, the constitution does a fabulous job of impeding change and protecting the power and privilege of the elite (now the wealthy elite). If however we wanted to have a constitution that provides what we have been promised in the country's mythology (Freedom, Opportunity, government for, by and of the people, etc), the whole thing would have to be rethought from scratch. In that case one would hardly be able to recognize the original document in the ideas of the new.

If you look at a Constitution more recently written you find things addressed that the Founding Fathers sort of missed.  South Africa put together a pretty comprehensive document.  Of course, they seem to be having a little trouble abiding by its more progressive components.  It shares many of our own Constitution's ideas.  Here's a few they added that reflect over 200 hundred years of political evolution- Citizens have the right to make their own decisions regarding reproduction. They have the right to form political parties and form unions. Housing is a right. Health care is a right. The South African Constitution is a Tea Party nightmare, but it has its fans including Supreme Court justice Ruth Bader Ginsburg.

The Constitution, in and of itself, is pretty damn vague. Updating it for changes in society would mean mostly ripping out stuff that turned out to be irrelevant (writing letters of marque and reprisal, the electoral college).  The basic idea of three branches of government is widely considered successful and is adopted all over the world. There are tinkerings, but none is considered radically superior. For the most part, powers allocated to Congress and the President are mostly consistent with what was written, though their vagueness has become a massive burden. Modern governments are expected to coordinate far more activity than they did at the time. You think the IRS is unconstitutional?  Hell, by that thinking, the FAA is unconstitutional, and there's now way in hell you're getting me in an airplane without it, or something equivalent. It might be nice to have Marbury v Madison actually coded up in the Constitution, but everybody seems to be doing just fine without it. Many of the various amendments could, in theory, be seriously reconsidered. Words like "arms" and "press" and "papers and effects" have altered drastically over time. Even "speech" has changed, given that the Framers were unaware that a vast fraction of the country is willing to believe whatever idiocy is pumped at them from the magic box. In theory, that is. In practice, the Congress is never going to pass a significant law ever again, much less the far higher threshold of a single amendment. "Rewrite the constitution" is in "pigs fly" territory.

I think it would need only a few basic changes to clarify and protect some aspects of modern life that are a bit outside of it's original time and context. One of the greatest things about the US Constitution is the elegant and very general language it uses. It is far different from modern "legalese" we see everywhere and by comparison is practically poetry. If we were to edit it at all it should be done in the same kind of persuasive and emotive language without trying to cover every legal loophole. Some examples I personally would like to see. We, the People of the United States of America, reject the U.S. Supreme Court's Citizens United ruling, and move to amend our Constitution to firmly establish that money is not speech, and that human beings, not corporations, are persons entitled to constitutional rights. No one should be discriminated against by another in any aspect of their life on the basis of any factor except for that persons ability to meet the essential requirements and or duties of the relevant job or task in question with or without accommodation. It shall be an unlawful discriminatory practice to wholly or partially for a discriminatory reason act based upon the actual or perceived factors including, but not limited to: race, color, religion, national origin, sex, age, marital status, personal appearance, sexual orientation, gender identity or expression, family responsibilities, genetic information, disability, matriculation, or political affiliation of any individual by any organization, governmental agency or individual where a significant and compelling need does not exist and no means of reasonable accommodation will correct this. We reaffirm the fact that all persons are innocent until proven guilty and shall be treated in that respect in all aspects of the law in all reasonable and practical ways. (recently the supreme court ruled anyone can be strip searched for ANY violation of law even minor traffic offenses and such things as eating on the subway[ there's laws against eating on the subway in some cities]. This applies even if the person was found to be innocent of the crime they were accused of.) The US Congress and State and local governments shall have the authority to legislate to enforce these rules as needed. The key here is the exact language of the constitution can be loose because the congress is tasked with enacting laws to enforce such concepts not the constitution itself. <sourced some from http://movetoamend.org and the DC civil rights laws. To base the wordings and or copied>>

One way of looking at this question is to look at how SCOTUS rulings have clarified/altered the meaning of the words contained within the original Constitution and its amendments. Some things that strike me as important: There is a limited right to privacy, which currently extends in part to a woman's right to make decisions governing her own body. I would argue that this right to privacy extends to sexual preference, as we now understand it from the standpoint of biology. Corporations are considered people, in contrast to the common-sense understanding that only people are people. Interstate commerce has blown the doors off of the tenth amendment. The rights to be protected from unwarranted search and seizure and cruel and unusual punishment have been severely curtailed by the judicial interpretations of the prerogatives of the government. Elections are subject to the court's political preferences. The right to vote is conditional.

Something like the War Powers Resolution would be put in, or even the Ludlow Amendment.  But building from scratch, you'd still want a commerce clause, a full-faith-and-credit-clause, a necessary and proper powers clause, a recommendation clause, a comity clause, an interstate extradition clause, a clause establishing it as supreme over state constitutions, and the no religious test clause. You'd probably want to avoid recess appointments in the modern era when Congress is almost continuously in session, so that wouldn't go in. You'd probably want to make pardons subject to congressional approval, if only to avoid them being used as a last-day-in-office favor to political friends.  You'd want to specify that "he or she shall from time to time" report to Congress on the State of Union in writing.  You would want to explicitly authorize judicial review by the SCOTUS, as well as establishing it as the court of last resort.  You would want to swap out the current text of the Second Amendment (this is what y'all came here to read about, innit?) for the current text of the Fourteenth.

Nothing. Except to include some of the Amendments into the body, The Constitution has been changed to bring it up to date and can be further changed if need be by: Both houses of Congress (the House of  Representatives and the Senate) proposes a constitutional amendment. This requires a 2/3 vote in each house. Once this happens, the proposed amendment is submitted to the states for ratification. The legislatures of 3/4 of the states must ratify the amendment, within a certain period if specified. At this point, it becomes part of the Constitution. A second method starts with the states, and was included to give states a check on overreaching federal power. The legislatures of the states apply to Congress for a convention to propose amendments to the constitution. If 2/3 of the states make such applications, congress must call the convention. Once such a convention is called, and amendments are proposed, the amendment(s) must be ratified by the legislatures of 3/4 of the states. This method has never actually been used to amend the constitution, but Congress has proposed amendments in response to threats by states to call a convention, likely to retain some control over the amendment process.

USA's constitution is based on justice, liberty, equality, freedom and God fearing humanism and these fundamentals were same then as today, so I think it would remain mostly the same. Note: I am Indian but a keen observer.