Honestly I would be fine if they just setup some kind of mirror so their posts would show up across different platforms. The biggest issue with then not using mastodon is just not being informed of what is going on. They don’t need to engage with people on mastodon just setup some kind of bridge.
I usually also backup the etc directory so if I had an issue I would at least have the config files from the old setup. This has already saved me a few times when I have really messed up configuration files.