rear 6x9's

on my '02 ls, as i assume holds to all impalas with the premium sound, the rear 6x9's are 2 ohm. most good speakers are 4 ohm it seems. any issue with changing the speaker impedance? obviously if im increasing the impedance it shouldnt be a big deal but will the power output be noticably lower, or will that amp underdrive the new speakers? any thoughts?