It would be impossible to change dates, really. The dates at this point have been nailed down for quite some time.
Change of date (even if there was another free date around that weekend) would result in many consequences: a lot of external logistics like private security, AV needs for events, etc. would need to change dates. I'm not positive but I think we would need to pay the convention a change of date fee. We would also need to deal with attendees who already took those dates off from work, or planned things around that date, or made hotel reservations, etc.
On top of that, what weekend is better? Summer is when kids are out of class and people take vacations. Every weekend is littered with conventions for months.