Being a Fae source, it wouldnt have to be a literal body swap, it could be more of a mental compulsion combined with glamour; the analogy would be More phantasm:Holomancy::[This]:Transmogrification. That could even be part of the secret they need to figure out to reverse it. It would sidestep the Soul mechanics and could reasonably alter mental capacities, memory, and even magic. There might always be somethign that could overwhelm it (True Love or Names or Strong enough Faith). Some of that could be locked away though, for example a Wizard might not be able to summon his Sight while he thinks he's somebody that doesnt have it; but if shifted into the body of another wizard, there wouldnt be eny mental dissonance to block the ability.
Also, Id suggest having a situation where the swap occurs randomly, and make them draw names out of a lot. That way they dont get a chance to get too comfortable in their new skin.