[sydney1]

Forward $old to $new

Setup a filter in $new email that labels all mail received on the $old address, so you know where you need to update your email address

Keep this until you don't have any more emails coming in with that $old label for ... however long you're comfortable, 6 months, 1 year, forever. Unless there's a reason you need to decommission the $old one, I'd just leave it there