John,
Yes, I have changed the fluid. It takes approx. 700ml of Dextron, preferably Mobil 1 synthetic ATF. I try to do this every 15,000km, I think the 4matic manual states a number above this. The tool required is a 14mm hex (Allen) socket. There are two plugs just like the rear diff which also uses the same size tool.
If you are interested you can join our own 4matic listeserver at :
http://marshallmayer.org/mailman/listinfo/4maniacs
then post to:
4maniacs@marshallmayer.org