Well, believe it or not. Despite following all the instructions nwelite and mrlimo mentioned above, CO still failed to upgrade me for trips that I purchased when I was a NW Silver, for which I am now Gold. I arrived at the gate 2 hours prior to departure, the first one to be on the airport standby list. The counter agent told me he absolutely see no reason how I wouldn't be upgraded based on my status. He could see that I'm Gold on his screen.
However, after waiting very, very patiently at the gate, only 3 of us (myself and 2 other CO Silvers) were waiting to be upgraded after everyone else boarded the plane. The gate agent then issued upgrades to the other 2 folks. Then, I approached the agent and he said I have to take my coach seat as he had just assigned the last 2 FC seats. After asking him some questions, he said I appear as a NW Silver, not Gold!! How can this be explained? Since I just turned Gold, I have no card to prove it to him. I was indeed very mad at this, but there's no point in arguing with him. Before the trip, I have called CO numerous times to verify my new status and they seem to be able to see the new status. As a result, I lost 2 upgrades. It was NOT a pleasant trip home at all! I can't explain my disappointment.
Anyway, I got home and I called CO again to have them wipe out my ff number and re-enter the number for my upcoming 2 trips. I shall see if it works this time.