I just changed my data plan from 200MB to 2GB a few days ago. The only thing you can't do is view your data usage until your next billing cycle begins.
This, apparently, does not work on an ipad. I just chose to "change" to the 2G plan, and it does not offer the option to apply the change retroactively. It simply says "great - when you use up your 250mb, the 2G plan will come into affect"
I tried to get an At&t rep to retroactively change the plan and she basically said "no"