Hi 
So my solution is this: in your possessed pawn blueprint destroy your current ai controller, then spawn a new one and possess your pawn. The issue with ai controllers is that you could not destoy them in blueprints by destoyActor node, so the work around is to use SetLifeSpan with a small duration( you could read the discussion here Ai controller can never be destroyed or be garbaged collected - #16 by acatalept). The way I would solve your problem is this:
Hope this helps