Hi Kathy:
You can change it, but this isn't something we'd normally advise being done, as there may be other unintended side effects.
You'll have to edit the Member's file. In your Members directory, find the file that corresponds with their new member number & change the member number in the filename to the original number.
When you've done that, go into your control panel and run the rebuild & reindex functions. You'll want to run all of the options on the Rebuild page, and then run the Search Reindex...
Note: Any posts made by the member since the new registration would show as made by an unregistered user.