How to change at Doncaster railway...?

ample time but you can get a direct train to London Euston from Manchester Euston to Kings Cross on the underground is only two stops

There is a large subway connecting all the platforms at Doncaster. You will have to use this. I can't remember the platform numbering but it is likely that you will have to use this subway. Go down the steps, turn right then go along and up the next set of steps. There are, however, plenty of indicator boards showing arrivals and departures and the platforms to be used and each platform has an indicator showing the 'next train' details. Incidentally, going to London from Manchester Piccadilly via Doncaster seems a heck of a way round, but no doubt you have your reasons.

12 minutes is ample time to change trains. Saturday evening is off-peak so things should be fairly quiet. As to last-minute platform changes, if they happen, they will be announced and passengers given enough time to move from one platform to the other. I don't know what it's like at Doncaster because I don't use the station. It happens a lot at Bristol Temple Meads which is a bigger station than Doncaster.

I can't understand why you need to change in between trains. Will your clothes be dirty after one train journey ? There are toilets at Doncaster station so you could always change in there. I suppose the 12 minutes depends upon what you need to change! Is it a full outfit or just your top. Most people who travel by train nowadays get from their origin to their destination in the same set of clothes.

I live in Doncaster.. unfortunately for you I'm not a fan of the train. In fact, I've probably used the train less than 10 times. Someone will probably help you out there if you ask (avoid chavs, they're more likely to beat you up and/or steal your wallet than they are to help you and Doncaster is infested with them unfortunately =/).