Well, the good news is that the new domain works and phpBB has been installed.
The bad news is that the database update failed. I started from phpBB’s backup feature, which produces a database dump of an impressive 42M. phpBB itself barfs on the file. And so does phpMyAdmin, which says it’s too big. And BigDump doesn’t run.
If anyone knows how to do this, I’d appreciate some advice. (I mean, someone who’s actually done this; I know how to Google too.) This is on Dreamhost by the way.
(The backup plan is to start with an empty board, but it’d be nice to do better than that.)