Here's how it works:
Scenario 1:
Switch from 900 to 450 minutes
Have 1200 Rollovers
After the switch you will have 450 rollovers
Scenario 2:
Switch from 900 to 450 minutes
Have 200 Rollovers
After the switch you will have 450 rollovers
The reason behind this is to prevent customers from paying for a really high minute plan for one month and downgrading to the lowest plan and using the rollover minutes leftover from the higher plan. You can't blame AT&T for this, they are trying to make a buck, too.
That being said, it is possible and in fact I encourage you to either talk to CS or to a store manager because they have the ability to restore SOME (not necessarily all) of these rollover minutes.