Hello to everyone.
Today, after reading all possible info about MKIII and MKIV NAV's I switched MKIII to MKIV NAV from 2004 X3 to 2002 X5 (with the 2004 NAV disk), without any problems, it work's. And YES they are interchangeable.
This is first step of my project of upgrading my X5. I think now I can upgrade OC to 09/2005 version, mine shows 4/1-63 it's not the latest one if I am not mistaken.
Second step is to put back MKIII (with the 2001.2 NAV disk)back in 2004 X3. One Q about this, will onboard computer recognize MKIII system and will this work?
Thank's to PEPATRICK for advise about swaping NAV'S.