You must repay the credit only if the home ceases to be your principal residence within 36 months of purchase. The full amount of the credit comes due on the return for the year the homes ceases to be your principal residence. However, you only have to repay the credit to the extent that you have a profit if you sell your home. Contact your tax advisor about the specifics of your situation.
Michael Saunders & Company
If you have an accountant who helps you file your taxes I'd call him/her and ask for advice.