when you lower your plan, you retain rollover minutes only equal to the number of minutes in your new plan. for example, if you had a 1000 min plan with 5000 rollover minutes and decided to switch to a plan with 450 min, you'd only be able to keep 450 of your rollover minutes.
EDIT: i assumed this was for at&t. i have no idea how it works for t-mobile, verizon, sprint, etc.