Like it says:
It would seem intuitive to just change it there and try again, but it's not actually editable.
To get to this point, I file-copied the original project, opened it in Bazaar Explorer, saw that it found the history okay, and proceeded to check out the project in its new home. After a minor bug fix, I tried to commit, and it did this.
This isn't a satisfactory answer for my original question, so I'll leave it open for one that is, but I've decided to abandon the migrated repository and make a new one with (partially) migrated contents instead.
I know it'll reset my version history, but I have enough other changes, having been distracted by other, higher-priority projects for a while and coming back fresh, that I think it makes sense to call it a new project that's only based on the old one.