We're running a test import from AVForums into xenForo tomorrow (getting a dedicated server for the job and running on a copy of the database).
What is the process for removing the xenForo indexes prior to running the import and how do I then restore the indexes after the import is complete?
Well, disabling the keys may help speed it up a fair amount - I don't know of anyone else that's done that for the import (just the search index rebuild) - but I'm really not sure. It will be a number of hours, possibly around a day!
Only a day is not so bad. Not good to have the forums down for a day, but not that bad either.
You'll have considered this, but is it possible to do an import from a snapshot of the database and then do an secondary, incremental import from the live forum? That way the bulk of the data is imported in the first pass and the relatively small number of changes which happened over the following day in the second. Then the forums need not be down at all.