Need to bring an OLD v13 system up to date

Taking a look at a new-to-me, old-to-them server. It’s old (set up several years ago), but has a lot of configuration that I would prefer not to have to replicate from scratch. It’s using OSS Extension manager so that will have to be switched to licensed mode. It also has an analog card (Sangoma). The machine is fully 64-bit so I don’t have that compatibility concern though.
How bad is this going to be? Just march through version by version, with a full drive image backup all along the way?

Sounds about right. backup, upgrade, test, repeat. As far as issues, I’ve had some go completely fine, some have gone awry, but all were recoverable. I never needed to revert to a backup.

Rather than attempt to upgrade in place, I’d instead consider doing a backup using FreePBX’s backup utility, then swapping drives and installing fresh, and restoring the backup.

I believe this is a faster, cleaner, and safer method. Worst case scenario if you run into an issue, reverting is as simple as booting off the old drives.

Whenever a client presents us a server more than one major version behind, this is our preferred method, and I believe it’s the one recommended by Sangoma as well.