We're actually prepping content for the migration a head of time. We're expecting about 2 days worth of posts to be misplaced, but not lost. Once the new site is live we will be leaving these forums up as an archive for an undetermined amount of time, though you will no long be able to post on them.
The reason for the 2 day gap in posts, is the nearly 2 million posts we are migrating take that long to convert from Xenforo to our integrated forum software. We tried to find several work around, but ultimately it came to the choice of shutting the site down completely for several days to move all the content and set up the structure we'll have on the new site, or using a backup from today to move everything to this point and having less down time. There are going to be quite a few changes to the forums, some are becoming obsolete and are being archived (visible but unable to be posted in) due to new features of the site, others haven't been used in several years and are being archived due to disuse, and yet others will be shifted in their positions from being sub-forums to headlining on their own.
We're hoping for no more than a day of down time, worst case 2 days.