sure its possible...just take out the old speaker and plug in the new one. it will sound a little better than stock but newer speakers would have more potential to sound even better with more power and thicker wiring.
if you plan on selling the car in the future, its a good investment to put in and take out later.
i am running boston pros all around with components in the front. even though i haven't powered these with an aftermarket amp, the mids and highs sound great to me. however, bass-wise they need to be amped to be satisfying. that's why i have 2 subwoofers in a sealed enclosure in the trunk to handle the bass.