Since I already had a phpBB2 installation, I tried the one-click to update it, and it failed. So I created a new subdomain and a new db and manually installed phpBB3 there.
During that installation, it asked me where my old stuff was, so I pointed it to the BB2 database, and the .old BB2 directory and it updated my new installation. Then I renamed the domain as .new and renamed the subdomain as my domain and went with it.
My manual BB3 installation took the place of the failed One-click BB3 directory, and is using the new BB3 database. I don’t know what will happen when they get one-click running again, but if BB3 has its own updating mechanism (a la WordPress), it won’t matter.