magnus,
Unfortunately, I have always just replaced the CF as they were easily obtainable and just sent the old one back after change the HWID on the AMC and downloading the license file again.
You only need SWA to upgrade the software version on the AMC, but you may be able to downgrade he 6.3 CF to 5.0 although I have never gone that far back on one it should be possible.
The other option is to get them to pay for SWA and upgrade them to 6.3 so they are at the latest version. As for the DB you will most likely have to rekey the whole thing.
The other option would be to forklift them to the new MiVB system that only supports 150 users so they can at least keep their phones.
Sorry,
TE