Somewhere else, I came across a help seeking thread where someone renamed a neighbourhood and it wouldn't show up anymore.
The standard answer to that in the early days was:  make a dummy in game with the number you want, so 'hoodmanager  expects a hood with that number.  Then quit and replace the dummy you just created with your re-numbered hood.  It'll be fine."  Of course we didn't know about the unique number issue.
I have some questions though:
- What kind of problem would a non-unique number cause?
- Is it safe to delete the 'hood manager file?  I used the trick with a re-install to fix flashing hoods in Seasons!
- when you add a sub-hood like Bluewater or Sedona, does the game allocate a unique number to that sub-hood properly?
I just re-name in a DOS window.  It's quick - apart from typing in the long file path.