YOur kind of looking at it back asswards.
Swapping in more powerful speakers means you need to provide them with more power. They have no power, they are just speakers. They handle power. More "powerful" speakers will just get the same power the old ones did and will loose bass output as they need more juice to properly move.
Any speaker that says "200 WATTS" in big letters on the box is pure bulls***. First off speakers HAVE NO POWER. Second a real 200 watts rms would desroy any such POS speaker. This is what real 200 watt RMS capable speakers look like:
http://www.bostonacoustics.com/ca_product.asp?ProductID=216&CategoryID=14
They cost around $1000/pair. But you need an amp capable of producing that power or they are wasted. The weakest amp I will sell them with is a JL audio 300/2 which is 150 wattsrms by 2.
SO to get more power, you must add an amplifier or a more powerful head unit. To date no head unit has produced more then 27 watts by 4. NONE. The 60x4 ratings are bulls***. So at best the most powerful head unit ever built only produced 108 watts total. Your car will only produce around 40 watts total regardless of what is hooked up to it as I mentioned since the speaker do not produce power, they just handle it.
Now to address your question directly, yes, you can keep the factory changer. Many have. You will simply need to add an external amplifier to the system to better power aftermarket speakers. IF you are not going to do this and stay to speakers around $100/pair as anything more will require too much power to be beneficial in your system and would be a waste of money.