you should go to the 6921 phone first and click on the "line/dn" deasociate it from that phone then go back to the phone,6921, and deleat it.
you can copy another 7945 phone and use your new MAC address, then click on the line/dn, when you add the extension all the information should come up, this way you don,t have to "rebuild" the line/ext information.