Best practice for going from v2.1 to v3.3?
It's been a very long time, the forum in question has been pretty much unmaintained since it has been setup in 2013, though it's still working properly and likely just needs a template and database cleanup (I guess a lot of cruft has been added to the database because of many deleted users = spammers).
Ideally I would want to setup a new site and import settings, users, posts and attachments, but it doesn't seem to be possible in any easy way, or is it?
So what would be the recommended way to bring such an old version to the year 2020?
Simply trying to upgrade with the latest version or incremental upgrades from one subversion to the next?
If going the upgrade route instead of a new installation, what would be the recommended steps to clean up crufty files and database entries?
In hope that that forum can be saved and brought back up-to-date instead of moving on to a different forum software (as some members of that forum suggested).