You can do that a little easier... First, delete the Phone for the user, then create the new 53xx set and give it the extension number, this will prompt you that a mailbox exists and ask you to fix the conflict or associate the new extension with the existing mailbox, then you can associate it. This way the user doesn't loose any greetings/messages they had. Typically you would have a user to copy from, and could copy and paste most settings from that user to the new one too.