here is the pdf on how to change the mechatronic & seals. However, the mechatronic unit usually is fine, the GKE module might lose it's memory and/or get corrupted due to low voltage. I think it was believed the mechatronic was going bad, and in the early stages of fixing the 1-2 hard shift, BMW & ZF might have thought changing the mechatronic unit will solve the issue (which it does, until the battery voltage drops again), but you're out about 1600$ to do that. The pdf is helpful if the square seals are bust and need replacing, which like every rubber they might.
6 Speed Mechatronic Replacement.pdf