I have a feeling the most likely agent for change in the Sun Belt would be if two or more conferences raid Conference USA that caused CUSA to fall below seven members (such as the Mountain West and the Big East).
The next most likely would be more I-A upgrades, which could possibly include Texas State, UTSA, WKU, and Missouri State. If the Sun Belt gets above 9 in football, I'd expect Denver, USA, UNO, and UALR to all be gone, and quite possibly all to separate conferences.
I seriously doubt that UNO, USA, or UALR would leave if the Sunbelt added more teams. The Sunbelt commish and the Sunbelt schools' presidents have told USA, UNO, and UALR that they are welcome for as long as they (UNO, USA, and UALR) want to stay in the 'Belt. However, it is common knowledge that Denver is no longer wanted in the 'Belt, and both the 'Belt and Denver have been looking for a more better geographical fit for Denver.