"During the Membership survey which was conducted in early 2009 and concluded with a random focus group being surveyed at the 2009 Johnson City rally, the VAST majority of responders voted to keep the rally in July."
I made the point in another thread that this was a few years out from the Lima Ohio rally which, by all accounts was miserably hot as well.
To all of those who are searching for a better month, I suggest that you skirt the earlier dates as the May/June weather tends toward the wet in a large part of the country and we are as likely to run into a wet one as we were a hot one this year.
While a September date might not be too bad, you start to conflict with several of the larger regional (and long standing) rallies - Finger Lakes over Labor Day, Vermont the weekend following, etc... as well as getting into the local, county and state fair season. Like the MOA rallies, fair folks start arriving early to set up and linger in take down so you'll likely find fair organizers may be hesitant to schedule in something like what the MOA does within a couple weeks of their own event. In effect you'll now be discounting OTHER parts of the country by moving the date to September to accommodate heat and the south west region.
It has been well documented that a "moving target" is a recipe for disaster. There is enough case study on how well established events, both large and small, have crashed and burned when this was tried.