AR-15 conversion questions?

I keep hearing that the ARs can be multi caliber. From what I understand you just pop the 2pins out take the upper off then put the new one on. The thing I dont get is how you can go from a 223 to a 308 since the bullets are different sizes and would need a different magazine and a different lower recever. Am I missing something? Should I just get 1 or the other or is there a way to get 1 lower and multiple uppers?