In a nutshell:
EXCEPTION FOR LONG-TIME RESIDENTS OF SAME PRINCIPAL RESIDENCE.–In the case of an individual (and, if married, such individual’s spouse) who has owned and used the same residence as such individual’s principal residence for any 5-consecutive-year period during the 8-year period ending on the date of the purchase of a subsequent principal residence, such individual shall be treated as a first-time homebuyer for purposes of this section with respect to the purchase of such subsequent residence.
Of interest: According to economists at Goldman Sachs, around 70% of all current homeowners would be eligible for the expanded home buyer tax credit.