I personally would see it as an opportunity to replace them with some better speakers than the standard ones, with better power handing, for a start. Why go to all the hassle of removing door cards, etc. only to replace like with like?
Kevin
Whilst they certainly can be improved upon, esp at volume, std fit ain't all that bad, with most of the distortion coming from HU amps...
I'm guessing the HU has already been upgraded as the standard one wouldn't be capable of frying the speakers. Just guessing though. :-/
Be aware that some decent speakers are less sensitive so need more power to achieve the same volume levels. Not an issue if you have an amp or a head unit with decent output, but might be worth thinking about if you're retaining the factory head unit.
You could either fit a pair of woofers and keep the factory tweeters or change for a decent set of 2 way speakers, although you'll have to find somewhere to hide the crossover as Martin said.
Also, the limiting factor in bass response is the limited volume within the door that the speakers have to work against. Aftermarket speakers will improve on the factory ones in this respect IME but don't expect miracles.
Best way is probably to go to a car audio shop and see what's available for your budget and what they recommend.
Kevin