NW Silver Elites get their upgrades at midnight on day of departure. However, for early morning departures or redeyes, it is conceivable that you will checkin BEFORE the system processes the upgrade (since you can checkin 24 hrs in advance of the flight). In a case such as that, you either have to forfeit the upgrade and go in coach, or have an agent manually reassign your seats at the gate (assuming that there are any left now).
Its a tough call to make. I usually try to get 5ABDF on the DC9s if I am stuck in coach, but if I get on early enough to snag that seat, I am effectively reducing my chances for an upgrade.
"The A340-300 may boast a long range, but the A340 is underpowered" -- Robert Milton, CEO - Air Canada