Short story: Exchange box died. Rebuilt with same host name. Old mailstore is lost. All mailboxes will have to be removed and re-added.
The only backups I was able to make were the locally cached OST files. Now that the Exchange mailbox is a different ID, Outlook won't accept them as a data file, nor can I import them.
I'm sure there has to be a way to either convert these to PST reliably, or import directly, or possibly modify the SID information to load with the current mailbox.