I have a Voyager Anniversary Edition, i.e., the electric blue before that was a thing. It came with OS version 3.5 but I suspect that the CPU board is still the old one (I can not change banks from the front panel).
Voyager production ceased in 2018. Moog Music has no spare Voyager parts available, InMusic closed the service shop, and support and upgrades for Voyagers are no longer being performed.
Once production stops and the warranty of the last registered Voyager expires, maintaining spare parts inventory is no longer a priority as warehousing parts is actually expensive. Moog Music is hardly alone. Some companies even destroy their remainder of parts.
The CPU board has no impact on the sound and the OS works with either version. The only gain with the new CPU board is expanded patch memory of seven banks each with 128 patches.
Changing banks is done by holding the Cursor button down while pressing the -1/+1 buttons on 3.5. You’re saying that doesn’t work, because it wouldn’t make sense to have different banks but be unable to switch between them?