Is worth it to replace only front and rear speakers?

[spkrman]

In most situations no.

Remember the puny power we are dealing with here, big speakers will require big power to make them worth it... otherwise the swap will yield improved quality, but much less output. Been there done that in many a car.

I would at *least* upgrade the factory head unit before swapping speakers... but aftermarket speakers are 99.9% of the time going to GREATLY benefit from an amp.