Change User in CCK User Reference field to another user

I have a cck user reference field on a content type. I have to delete user "John Doe" who is referenced and change it to another user "Jane Doe." How can I tell drupal 6 that where ever "John Doe" is referenced change it to "Jane Doe"?

Thanks in advance

UPDATE: By the way, I'm willing to delete user "John Doe" if necessary.