what do you mean upgrade to a gts? you can trade your car in and buy a gts. any other way is unrealistic to do it (you'd need an engine swap, tranny swap, etc...lots of other, smaller things).
if you just bought a used gt, then resale shouldn't hurt you very much (you should get almost what you paid for it--minus tt&l). if you want a gts. i suggest selling your car while you still have so few miles on it.
also, do not get an '02 gts. they have rev-limiters. you can get them fixed though..so if you get a good deal then i'd go for it. but otherise just buy any other model gts instead of the '02s.