Slowman wrote:
we have waaaaay too much data to risk the migration.
It is not really a risk. Existing database can be left as-is, and the data migration builds new data structures for the new forum. It's a copy rather than a move. This can be done by restoring backups of the existing databases and then working on that copy. That way the live forum is untouched.
You can even test drive the new forum by installing new software, it has new empty(ish) databases right? Then do data migration. The new forum software probably has tools to do that. Insert the existing data into the new databases. You got a copy of Slowtwitch! Running on new software. See how it all works. Work out issues, bugs, etc. Then do it again to go live, but this time shut the forum off or at least make it read only, so no posts are lost in the migration.
Edit: path to back off the changes and go back to the old forum should be available any time in the upgrade process. The parallel forum with a test migration from backups, then a live migration, will do that. The only downside is you'll temporarily need double the storage (disk space) but disk space is cheap.