Home Buying Credit Expansion Act - Amends the Internal Revenue Code to: (1) extend the first-time homebuyer tax credit to all individuals who purchase a principal residence (currently, only first-time homebuyers as so defined); (2) extend such credit and the waiver of recapture requirements for such credit through 2010; and (3) expand the election to treat a purchase of a principal residence as made in a prior taxable year for purposes of such credit.