damn i never thought about that before....i'm a firm believer that kyle can't quite enjoy cartman changing unless he's the one providing/participating in said change so what i imagine is that kyle would've initially been cool with this new cartman but progressively he would've gotten bored and started theorizing that cartman is just pretending and hasn't actually changed. just waiting for cartman to screw up so he can point out how much of a phony he was. you know, just usual kyle stuff.
you know what would be a fun alternative though? if kyle got to be the one disciplining him, like going tsst! at him and watching cartman grovel in submission. i think kyle would enjoy effortlessly making cartman behave which again goes back to my idea that kyle wants to be part of any progress in cartman's self or it's a bust