This sounds more like a mind-games type of concept. I can imagine a CAP that hinders particular OU Pokemon (I dunno, we'll pretend it's something that gets use out of Prankster) by worrying the opponent when their Prankster is exchanged, enticing the opponent to switch out for an offensive presence to start hitting this CAP. In that case, this concept would have to have a coverage move to punish Prankster-mon's most common offensive team members upon switching in.Mummy is an option, but it's still just useless if there is no contact (not physical, contact moves specifically), and decent at best if there is contact. And what is "a CAP that fixes the problem" supposed to entail? If you are referring to a custom move/ability/the likes of, afaik that's still not allowed. If you are referring to making a Pokemon that is so good to carry the concept, then that runs the risk of losing focus and creating far better options for the CAP to run over the ability changing concept. You still haven't addressed my concern that switching out basically invalidates your entire concept, and the fact that for it to succeed, you kind of rely on the opponent not knowing what they are doing. Please explain how you can catch someone off guard with this concept to make it work when everyone knows what the concept does? It'll help me, and by extension CAP community/mods, understand where you are going with this. This can be done, but it'll be difficult to keep it balanced, focused, and still succeed.
However, I don't think anybody thinks like that. This CAP seems too likely to become the guy that asks for switches without forcing them, and you could take something like MegaZam, who doesn't rely on his own ability anyway, and hit this CAP before it does any of its setup nonsense. I would say use Arena Trap and Entrainment on a Pokemon combined with ridiculously high speed so you can pick off that one Poke that has a defense-oriented ability, but there are vastly better and more viable mind games out there, really.