League bylaws are not legally binding; however, it is professional courtesy to honor the agreement.
I believe if they left the league sooner, they would need to pay to break the schedule contracts with league members which would technically be remaining based on the 2-year rule...which would be quite expensive. If the NLL gets an 8th team this summer, I doubt either side would be upset with moving the timeframe up, especially if that 8th (or 6th, 7th, and 8th) end up a cause of the dissolution of another conference and are free to do as they wish.
I still just can't get over how much Whitmer their pants with this whole thing.