The dilemma is that if you want the dealer to suck the fluid out of the torque converter,
it will cost much more in labor. If you do a 30K+15K+15K+... schedule, gradually, the fluid gets replaced. On BMW forum, some experts would argue that "don't flush it". Some debris may get dislodged from the gears and start to cause troubles after the fluid change (some actually reported problems like those after that flushed the fluid). Therefore, people on BMW forum came down to a good compromise (if you will) to just do "drain and replace" on 30K+15K+15K... schedule.
Those who kept this fluid change schedule had not reported any tranny problems
as far as I know. I no longer frequent those BMW forum since I sold mine last year.
My friend's BMW 323 had tranny failure at 55K miles (warranty is 4yr/50K only).
When the mechanic opened the floor-pan of the tranny, many metal debris (size of rice grain) fell off or stayed on the magnets. An awful scene to watch. The repair costed him $5000 with a refurbished tranny. I knew for a fact that he did his part as a owner maintaining his vehicle.
So, Life-time fluid is a scam to sell more vehicles. If you want your tranny to last
longer than 100K, you better not forget about the tranny, which works very hard for you at any speed.