Yes you can upgrade these speakers without any issues unless you car uses a premuim sound amplfiier in which case you MAY have to bypass that...
EDIT:
Honestly.... if you are to start upgrading to aftermarket parts... your first upgrade should be a deck... it will make your stock speakers sound 10x better beacuse it will amplify them and cleanup the signal... A deck should always be the first upgrade... it also prepares you for any future upgrades planned, and makes them easy to work with.
Second Edit: Pyro is wrong if you have a premium sound amplifier... your car may be wired differently
THIRD EDIT:
Pyro.... most premium sound systems dont use standard speaker wire for EX: in my audi the premium sound amp had 5 wires that connect to EACH speaker... not 2... 5 its a different connection.