I disagree with Roach, sometimes a change of scenery is what you need as a fighter but I will say that nothing is going to change the fact Khan doesn't have a good chin.
I know many people say you can work on your chin but in all my years of being in boxing, I have seen no evidence of this....NONE.
You can strengthen your neck muscles and all that but I have seen nothing that shows me you can take a better punch. It's just one of those things that guys are born with. Khan needs to be a better defensive fighter for sure but he needs to avoid the big shot, it's just that simple.