[Edi]

The unique id is kept. All magics are kept. Just the monster type changes. I had one LA Ulm member of the Second Tier pick one up in a battle and he later changed into a skinshifter. Kept his magics, so he could still research. Useless in combat due to the autoberserking, though. The transformation requires a failure of an MR check and it presumably forces a check every turn. So eventually the transformation will occur.