I just upgraded my front speakers to Boss 4x6's and I had the premium 7 speaker option in my '02. I kept the speaker enclosure and just cut out the plastic to fit in my new speakers perfectly. I used just a multi-tool hand saw, really simple. My new speakers sound way better. If you are worried about taking out 2 speakers and replacing with just 1 4x6, don't. It sounds better. I have no regrets.
Also think about upgrading the Stock subwoofer. I added the Kicker 6.5 compact sub. Big improvement.