[vtl]

You cannot rewrite the ISN in the MSD80/MSD81, as they use an Infineon Tricore ECU, where the ISN is stored in a write once area of memory. The CAS uses an ancient series of Motorola CPUs, so you can just read and write the entire memory with many tools available off ebay/aliexpress. Typically the ISN from the donor DME will need to be written into your CAS, and your EGS will need to have the ISN written into it as well, otherwise the car will start but not go into gear. Not sure if this is achievable by just flashing it with ISTA/P or WinKFP, never had to do it myself.

EDIT:

Just spoke to someone who replaced the entire transmission from a donor car including new TCU it still works so might not apply to n54 and 6hp (this was on a N54 135i with dodgy transmission)