I am the owner of a 1994 Volvo 850 sedan. I know it does not get 18 mpg now but may have been estimated to get more than that when new. I have a 2010 Toyota Prius ordered. I am trying to figure out if I should wait to buy and to trade in the Volvo if it even qualifies for the voucher. If I purchase the Prius now will the bill be retroactive? Thanks. Claire