Someone said the older plug in style over heats so they were phasing it out. The new one hass a reduced output to fix that. I have the generation 1 umc.
The original UMC, such as the one that came with my early 2013 Model S, could charge at 40 amps, but there were some fires caused by overheated plugs. Tesla sent out new 40 amp 14-50 adapters to owners that incorporated an internal thermal fuse.
Still more fires, so Tesla sent out whole new UMC sets that were limited to 32 amps and included a new 32 amp 14-50 adapter. They asked owners to return their original UMCs, but few if any actually did. (I didn't). I believe this was the result of an investigation by the Ontario Electrical Safety Authority.
So there were actually 2 iterations of the "old style" UMC.
When I sold my 2013 Model S, I delivered it with the new 32 amp UMC and 32 amp 14-50 adapter plus the original 5-15 adapter than came with the car. I kept the old UMC along with both the fused and unfused 40 amp 14-50 adapters and the 5-20 adapter that I had purchased separately.