There is a conversion. It requires headlamps, bumper, fenders, hood, grille. I also believe it includes the radiator support and some other things. It would probably be easier and cheaper to sell/trade yours and get the 04/05 honestly.
If you can find a guy with an 04 that wants to look like an 02, it's actually not that much labor. Buying the parts though would be more money than it's worth compared to just selling it and buying a newer model.
Note though the tail lights etc are not easy to swap. I would recommend not doing the rear. They don't look THAT different anyway.
then buy an 04 why would you want a bug eye (best car of the year) to look like an 04 each Impreza year has something different and unique about them. Dont try to make a 02 look like 04 respect how they look and work with it. sorry to be harsh but its that simple.