I tried the old 50MAR code that II sent me multiple times over the past few months and that didn't work. I also called II and asked about any promotion for Gold / Platinum, rep checked and said "nothing available on my account".
I wanted the upgrade for the same reason (just traded a Marriott Grande Vista studio for a Marriott Grande Vista 3br in late June). I had to bite the bullet and pay the full ride for the Platinum upgrade. It cost me $139, but saved me $123 on this transaction. Even though trading a studio for a 3br is pretty awesome, the fees REALLY add up!