I have replaced the speakers in my 05 Kountry Star twice. First time I went with decent quality 4" speakers and they really sounded poor. If you want anything more than noise then I would suggest a bigger speaker. However, clearance above the ceiling is an issue. So I did a lot of research and found HTD speakers (link below) had suitable dimensions. So I visited their facility to listen to them and they sound pretty good, especially at only $65 each!
HD-R65 In-Ceiling Speakers
I was able to simply pull out the existing speakers, cut a bigger hole and install the new units. I did not try to replace the existing wiring (which is not very good quality speaker wire) because it seemed like it would be a lot of work.