Firstly there is no drain plug on the diff so you have to suck out the old fluid through the fill hole. Secondly depending whether you have a manual or an auto transmission will dictate the ease to which this job can be done. Auto transmissions being a lot harder to do. Given that the transmission will probably have the accursed plastic pan which are prone to cracking at the 60k mile mark then one should do the whole lot.
I agree with the sentiment of no such thing as lifetime oils and BMW will acknowledge privately that this is 100k miles........
Personally I would change these oils at around 60k miles. It needs to be remembered that todays oils are of a significantly better standard than oils of old, so maintaining or changing the oil somewhat earlier than the supposed life-time should be a good practice. Especially if you are keeping the car or you have purchased it with this kind of mileage on as no doubt it will need doing as many people do not do it!!!!!!
Does my bum look good in this !