I'm probably talking too soon (something will die), but that master database switch-over just went perfectly to plan. (well, 'cept the end where I forgot to load the newest version of BML on all the servers... but that was an easy fix)
I'd expected 30-50 minutes for everything, but it was done in under 20.
Now we got the super-beefy old master database to do fun stuff with. Will keep you posted on that. :-)