Okay, I'll just give an example.
Ex. I send out Cofagrigus, My opponent sends out Bisharp, Bisharp uses Night Slash, Crits, And my Cofagrigus is down, but he got the ability Mummy.
Now if my Archeops came in, and uses something like U-turn, and got Mummy, but he switched out.
Would he still have Mummy when I switched him back in?