I was Gold, and on a whim was calling MR for something unrelated and casually said, "Hey, can I have Platinum instead of Gold?"
I was informed they would enroll me in the challenge of 18 stays by Nov 30 and I'll have Platinum until 2013. In addition I was given Platinum within 2 hours during the duration of my challenge.
You may attempt the challenge on an annual basis per the MR agent. Call now and you'll get the remainder of August in addition to Sep-Nov to complete.
Damn shame I had 2 stays 3 nights before I called... They will not backdate at all.