Sorry, I got confused about what you were trying to upgrade. Yes, you can just buy new map disks. You need the CD version of the "High" disks. BMW still sells current map CDs.

Here is a post that explains how to figure out what your nav computer OS version is.
Here are the steps for the Service Menu I found somewhere

The newest navigation OS available for your NAV01 is 7.3.2. If you already have this, then it will not update.

__________________
SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input