That certainly is one way. BUT, you might be able to just plug the new phone in. There's a good chance it will boot up and eventually ask you to register it...if so, ### and the extension will get you all set.
This won't work if:
1. you don't have Mitel option on your DHCP server (if this is true you'll need to manually enter system IP into phone set, let us know and I'll tell you how)
2. Default replace code ### has been replaced or is disabled (go to SYSTEM/PHONE RELATED INFO, see PIN REPLACEMENT fields)