So as you can see, this didn't end up happening.
There seems, I think, to be an issue with upgrading our database, but I'm not completely sure.
@Indie and I have agreed that it is not worth the downtime either of us trying to figure it out, so we are going to go to Xenforo and have them do it for us. They will know how to resolve the issues, and if they don't, it gives a target that isn't me to shout at.
We will be in touch with them soon, figure out a date, and let you all know when this will happen. It will involve downtime again (though hopefully significantly less than this attempt).
And of course, finally, sorry I wasn't able to get it all working smoothly, and forcing you to be productive for a day.