I agree that allowing address to be visible from the old nick could make things easier, but my question still stands...
Why not just get the address info from the new nick? It's easy enough to do
$replace($address($newnick,5),$newnick,$nick) to get the entire address with nick for the old nick. Yes, if the nick/newnick was something that was the same in ident and/or host, it would replace extra things as well. If that's a concern, you can also use
$replace($gettok($address($newnick,5),1,33),$newnick,$nick) $+ ! $+ $gettok($address($newnick,5),2,33).
Obviously, if $address worked with the old nick, this would be even easier, but it still isn't too difficult to do.