I have researched this issue a lot on various forums. My conclusion is to change the auto trans fluid every 50,000 miles including filter. If one is already way over that, the post above suggesting to just change just the filter and trans oil in the pan makes sense to me.
My 05 3.0L auto calls for a change at 100,000. I did it at 58,000 for about $350 at an indy. It currently has 70,000 miles. Given the potential for catastrophic repair issues on the X5, I am hesitant to keep one beyond about 120,000 miles or less. I would never buy a high mileage X5 unless I knew that it had been very well maintained. Mine was a CPO purchased at 44,000 miles. My 3rd BMW, all purchased used. So far, because of circumstances, I haven't kept one beyond 100,000. I am "on the fence" about every owning another one because of reliability and maintenance costs. There are a lot of great cars out there. But, none drive like the BMW.