Any differences? Really? That’s your first question here? Look, there are dozens of really good people here in the SS forum who are willing to share a wealth of information, but few have any interest in spoon feeding here. So, to get the greatest amount of your questions answered, I’d encourage you to spend some of your own time familiarizing yourself about SS and the three ships you note. The SS website will show you all the detailed differences. Then return here with specific questions and you’ll get a plethora of advice, because the ships are quite different.
Yes. If space is available upon embarkation, you can pay the fare difference. I saw it happen onboard before, but it cost the upgrading couple more money than what they spent on a Vista booking.
Also, others have had luck making a “guaranty” reservation and getting upgraded to a balcony. This method is rare relative to the thousands of passengers who sail.