Just wanted to give a data point. I got instantly approved last Friday for the AA Platinum MC. History is as follows:
* May 2012 - Instantly Approved for Citi AA Personal AMEX and Personal Visa
* Dec 2012 - Got Citi AA Biz (Can't remember if instantly approved)
* Jun 2013 - Downgraded Citi AA AMEX to Citi AA Bronze
* Jan 3, 2014 - Instantly Approved for Citi AA Personal MC
Before applying, I was wondering whether the downgrade to AA Bronze would be treated as a new account/application and if it would result in the clock being reset. Apparently, at least for me, it did not matter.