replace them. two options: 1)nice set of component speakers for the front and a 2 channel amp for them. leave the rear speakers alone. 2)replace all the speakers and get a 4 channel amp. but you probably will find you want to replace the stock speakers as once you add sub/s you'll find it hard to really hear the words. depends on if you like rearfill also. most people don't amp the rear speakers. if you have people riding in the back though alot you might want to. post a budget for everything and you'll get alot of help.
Kicker ZX350.2 with nice CDT or a/d/s 6.5 component systems....unhook the rear speakers or just hook em to the head unit and then get a Kicker ZX750.1 and two Diamond D312D2's...That'll sound sweet