Right -- but as soon as you go onto the waitlist, your reservations will be split, which means, if this is an outbound flight, it will be possible for you to be upgraded separately in advance on your return flight.

Unless there are more seats available than names on the upgrade list* about 45 minutes before departure, I wouldn't bother.

* You can normally subtract anyone who appears on both the standby and upgrade lists, as they're likely non-rev and therefore likely to be behind you on the list. Just about everyone else will be ahead of you, though.