With Ultimate Pack you're on version 4.07m. The "Il-2 Sturmovik: 1946"-CD contains version 4.071m and a patch to version 4.08.
So you'll have to (and exactly in this order):
- buy and install "Il-2 Sturmovik: 1946" (make a backup, just copy the directory)
- Execute patch 4.08 (in the CD) (make a backup)
- Download and install patch 4.09b1m (make a backup)
- Download and install patch 4.10m (make a backup)
- Download and install patch 4.10.1m (make a backup)
Then download and install UP RC3
Artist
Edit: Added links to Updates