You can tap off the rear speakers if you use the proper rated preamp adaptor. This adaptor will change the speaker output level to RCA outputs. Hooking it will not have a factor on the origional speaker sound.
This would be the correect way although some amps will accept speaker line output as the source feed. We there is a amp driving the speakers it is better to preamp the output. Most all amp manufactures have filters built in to reduce noise on the RCA inputs.
Changing the speakers out can go either way. Trying to install speakers that require a larger baffel to work in the specific opening would hurt bass quality. The mids and highs would not suffer.
James