I've read several posts on this. if you plan on changing tranny fluids, do so early (around 50k kms) or just leave it in forever.
Also, when bmw says lifetime fluid, they mean lifetime of the tranny (anything between 100k and 150k) and not lifetime of the car.
And at 100k +, do NOT change your step tranny oil. I've heard on E46's which ours is based off of and uses the same engine/tranny, that when they changed their oil at higher mileage, soon came catastrophic tranny failure. so either you change it when new(under 50k) or leave it in for ever until tranny fails.