hifi Stereo Question...impedance, amp, rear shelf speakers

[Chard]

I have a 2000 Boxster S with the following stereo options:

M696 CD Radio "Porsche CDR-220"
M490 Hi-Fi Sound Package
M680 Hi-Fi Audio System with Booster
M584 Storage Box above Engine

I checked my factory amp and it is 6 channel. My factory head unit is the Becker CDR 220 and I have dash and door speakers. There seem to be many variations of stereo configurations.

I want to replace the dash and door speakers. I was under the assumption that the speakers are all 4 ohm since I don't have the BOSE option, but when I took apart my driver's door panel, I saw a sticker that said the door speakers are 2 ohm. I haven't had a chance to take out the dash speakers, but I already purchased a set of Focal KP100s, which are 4 ohm components.

I assume the rear shelf speakers are powered since I have a 6 channel amp (shocking because I can barely hear anything coming out of those). Does anyone know if those are 2 or 4 ohm?

Can I change the door speakers to 4 ohm and still retain the stock amp or must that be changed?

I am planning to change my head unit too.

Thanks in advance, new to aftermarket car stereos.