See https://www.irs.gov/taxtopics/tc611
Acceleration of repayment. In general, in the case of a home purchased in 2008 for which you received the first-time homebuyer credit, if you dispose of it, or you (and your spouse if married) stop using it as a principal residence in any taxable year during a 15-year repayment period, the credit repayment is accelerated. Similarly, in the case of a home purchased after 2008 for which you received the credit, if you dispose of it or you (and your spouse if married) stop using it as a principal residence within 36 months from the purchase date, the credit repayment is accelerated.
If you're subject to an accelerated credit repayment, you must increase your federal income tax for the year of disposition or cessation of use by the amount of any excess of the credit allowed over the sum of the additional taxes paid under the credit repayment requirement. However, there are exceptions.
