Repayment of First Time Homebuyer Credit?

We bought our first home in December 2008 but had to convert it to a rental when we had to move out of state for my husband's change of station (He's in the Navy.). I just received a notice in the mail that we had to pay our credit back. Is there a military clause with regards to that? Do we have to pay it back fully this year or do they accept installments? Thanks.