Probably too late for the person who commented, but one way you can achieve this is probably something like:
Set up an AI controller variable in your actor, then save a reference to it on Begin Play.
Under “Pawn” set a default AI, have the Auto Possess AI set to “placed in world or spawned”, and set up the “Unpossessed” event, which you use to have the AI Controller reference you saved reclaim control of your actor when your player drops possession of that actor.