[ Normally, this is the kind of thing Spike hates. You don't mess with magic — or whatever this is — and you don't just switch bodies on people, but the aggravation this has caused Jack more than makes up for it. Besides, heartbeat, body heat, air in his lungs..? Pretty nice perks after a century of being a corpse.
He can be found sauntering around, either trying to pass himself off as the real deal in an effort to ruin Jack's reputation or thinking up ways he can mess with his body. Why not? This isn't a problem unless it turns out to be more permanent than a day. ]