IIRC the process is to turn them into a mudcrab, then wait for them to turn back into an NPC instead of killing them.
1. Save first
2. Dismiss the follower
3. Before they can walk away, hit them with the Wabbajack
4. Hit them with something else, like a punch and they will revert, at the level they should be.
5. Rehire them.
Note, this can cause your follower to become hostile, so that is why you save first. On the PC, you can do this in the console by Enabling, then immediately Reenabling the follower and there is no issues with them getting hostile.