question about taxes

I came USA with J-1 visa on December 2002, based on the policy between China and USA, I don't need to pay federal incoming taxes for three year. last Nov (2004) I changed my J-1 to H-1b and I am sure people also don't need to pay federal incoming taxes for three year if he came USA with H-1b visa. Now my question is whether I need to pay the taxes of Nov and Dec 2004 since i changed my status? is anybody having the experience like me? any response will be appreciated!