Yep, you can retrain royal knights if you have a Royal Court?... I think (the first level, which upgrades to baronial). And knightly retinues replenish automatically with time, the same is true with Ghulam bodyguards. I remember a lot of times when I was trying to do some genetic clenasing by trying to kill my bad princes. They're tough to kill because their retinues keep replenishing.

This is also true with princely Kataphraktoi and Boyars... I think...