Can I turn a primary FreePBX 14 server into a backup?

If they don’t have any commercial modules, I’d use the conversion tool home2.freepbx.org

This way, once you finished migrating, just swap the IP’s and enable the Trunk.

Note: a few directories are not copied over, but can be easily done with rsync. It takes another 50 seconds…

See what’s not copied over, here:

Then you can setup the beckup server to be a warm spare.

https://wiki.freepbx.org/display/FPG/Warm+Spare+Setup