Just for your info. If you don't have a functioning board even as old as it is. Upgrading would not resolve your issue you would still be starting over with a net new board.
So I would say worst case. No matter how old your backup is. Restore that backup and live with what was lost in between then.
Classic has been gone for a long time. It is only supported to upgrade to the current version.
There are a couple of people here that still use it. But there are a lot of security issues with Classic especially the 5.x versions.
If you ask for help here. That person will need FTP access to your site and or a copy of your most recent backup.
