I have a BMW 728 1999/2000 model with the older style sat nav - 4:3 ratio not 16:9

The software is pretty clunky - no postcodes etc. I have been offered a 2006 CD and also a V27 firmware disc but I am nervous that if I put this disc in the CD the whole thing will stop working and I won't be able to use it at all. I doubt if BMW will tell me the answer - can anyone on here tell me?