I have had this happen, but if they stick around, in a couple of months they can figure it out ("Pain is the best teacher but no one wants to go to his class"- General Choi) Once they figure it out they can do really well. in a relatively short period of time. Hardest part is breaking old habits as opposed to developing certain striking skills. Same with ITF people who go to a place that allows leg kicks. Seg waying to a grappling art is a transition that takes a lot longer to become proficient.I think that is largely because you don't train TKD. A high level KKW player could be in real trouble were they to go to a non-KKW school, where people will punch you in the nose if you spar with your hands down. But they're still a high level player.