The procedure typically used is to collect the fluid when changing the filter, put that back in and top off with new fluid. A flush and fill is a drastic change likely to cause issues. (Something about adding a large amount of new cleaning agents).

One of the members on xo has been using this method and has 360,000 miles or something so it’s the method I will use on my two x5

__________________
2011 E70 • N55 (me)
2012 E70 • N63 (wife)