If I pay off part of a interest-only 10/20 loan before 10yrs, will my monthly payment then go down? Or is re-amortizing a 30-yr fixed better? (I'm approved for a cash-out refi. Cash will buy property zoned multi-family worth 2x loan amt.)

As always in situations like this, the answer is "it depends". Your 10/20 interest-only loan implies that you will have the same principal balance at the end of 10 years as you do now. If you make more than the minimum payment, than the principal balance will decline, and the payment when it converts in 10 years will be SMALLER than it would otherwise be, but it's unlikely that it will be smaller than the original interest-only payment. Many factors go into the decision about what kind of mortgage loan will serve your needs best. I recommend consulting a mortgage broker, or you can e-mail me (see my profile) and I might be able to give you some general advice (I've had about 8 different mortgages in my home-owing career, so I know them well). Good luck