Conventions cost A LOT of money, and I would guess you get diluted participation because they're so frequent. Alpha Xi Delta uses a very similar structure as described by KD and Phi Mu. Described a little differently, every other year is for everyone and the alternate year is just for collegians.
I would say you probably need to devote your next convention to updating the bylaws and then move forward. But there's no way the change won't suck or cause blow-back. If you're ready with how many hours at what hourly rate it costs to plan a convention plus the other costs not recouped from attendance fees, and some will come around. The others can pony up or shut up
