We change the tranny fluid at 50-60k.
The choice of fluid interval is based upon the results of an extended history of failures of BMW transmissions with lifetime fill fluid. They have been out there longer and there is a very tight tech group reporting.
The life of the fluid is 100k. If you wish the life of the trans to be greater, it stands to reason to get it and all the by-products of road wars (dirt, moisure, clutch, metallic wear) out before they become the problem.
From the trenches...
Bosch Master, ASE Master, L1
33 years MB technician