What is the axel to crown on a BMW fork? If it's longer than 368mm, then your HT and ST angles will become slacker.
IIRC it has around 405mm and 38mm fork offset
also 650 does screw up your steering hardcore. Im having to get a longer fork to deal with that because it is cheaper then a new wheel but it took a lot of math to figure out the best fork to put me back to a normal trail measurement.