Unless something drastically changes at JMU, JMU to the Sun Belt
has about as much chance of happening as VT joining the Pac-12. FWIW in the mid-2010s the Daily News Record (DNR), which is Harrisonburg's local newspaper, did a series of interviews with JMU's President and AD during the Summer about the future of JMU athletics.
One of the topics that is whether JMU football would ever consider switching subdivisions given that they had recently announced plans to expand the capacity of Bridgeforth Stadium. Both men confirmed that JMU had been approached by both Conference USA and the Sun Belt about joining the respective conferences. JMU's AD even confirmed that ODU had been lobbying hard for the conference to include JMU. However, both men confirmed that JMU declined the invitation citing neither conference was a "good fit" for JMU.
Since I grew up on JMU athletics because my grandmother had worked there for a number of years, I learned quickly learned that JMU's motivation for deciding which conference it wants to be affiliated with is based on 1 thing; a conference's ability to increase JMU's exposure with recruiting out-of-state regular students. Translation - JMU wants to be affiliated with a conference that has a footprint in the region(s) that it gets the majority of its out-of-state students which is the Mid-Atlantic Region and New England Region. Since both Conference USA and the Sun Belt have a Gulf Coast/Southern footprint, it makes sense why JMU thought neither one was a good fit.
I believe JMU's ideal situation is to be a member of a "northern" conference as a full or football-only member. If JMU is a football-only member, then it would prefer to have all of its other sports be a member of the Atlantic 10. The Atlantic 10 gives JMU the necessary exposure in the desired regions for recruiting out-of-state students and it would give JMU the opportunity to reunite with former instate CAA members/rivals (GMU, Richmond, and VCU).