If you're going to switch your stock 2002/03 tail lights to the stock 2004/05/06/07 tail lights, you'll also need a 2004/05/06/07 rear bumper because of the little half circles that protrude downward on the newer model tail lights. The 02/03 rear bumper is cut flat for the flat-bottomed 02/03 tail lights and won't accomodate those circles.
You can always have the 02-03 rear bumper trimmed to accommodate the 04-05 "bubble". I've seen it done and when done right and carefully it does look good. If you cut the "circles" carefully, sand them and have them painted, it will look OEM.
The 04-05 are the best looking Impreza tail lights ever...
I think he means putting stock 04 rear wrx tails on his 03 , all he has to do is cut the foam from under the "bubble" (bottom inside circlular light) and the light sticks out about a cm and doesn't rub paint
hope this helps