Nothing is getting wiped. Let's be clear on that.
I'm dumping everyone's database into a file, then dumping that file into the database on the new system. I'm keeping full/exact backups of the old system, so if something IS wrong, I can redo the import again until it's correct. Worst case scenario is some issues with sort-order based on language. The new system is defaulting to UTF-8 for everything, whereas the current system is (for some reason I never figured out) using case-insensitive Swedish. For any data in the latin-1 character set, it shouldn't change anything, but if you happen to have other character sets in your data, AND you're relying on the default sort order, things may change around. Really, this shouldn't be an issue for anyone except people who wrote their own code AND aren't using latin-1 characters (e.g. russian or asian characters.)
As for the lock, that'll only be at the server move, and probably only for a few hours. You'll see a notice here when I ship the server, and probably another once I've got it online. Only then will it be locked while I physically copy stuff over. People visiting any site here will see a big notice saying exactly what's going on.