OK, I've come up with a loose timetable of the work I'll be doing to switch over forum softwares. Actual times are up in the air and totally dependant on when I wake up on Saturday morning, but I hope to start at 0600Z (sorry to our eastern European and far eastern pilots... you'll be the most affected...) Anyhow, here's the task list.
1. Disable non-admin access to both forum softwares
2. Switch round the URLs
3. Migrate content from old forum to new forum (this takes the time)
4. Post-migration tidy up (this also will take some time)
5. Test new forum on old forum URL
5. Enable non-admin access to new forum
6. Test new forum again, Just In Case ™
As you can hopefully see, there's plenty of space for rolling back to the old forum in that (not that we'll need to).
I'll be as quick as I can, not wanting to keep the forum turned off entirely for any longer than I need to, but steps 3 and 4 are "as long as it takes" steps, so I can't begin to guess on when I'll be finished and you'll all have access again.