I don't see how anything could be a step down from the Big South. They are a bunch of tiny, mostly private schools that are always at the bottom of Div I in bb.
Radford's move does seem a little strange as the Atlantic Sun is losing some of their better known programs and seems to be turning itself into a more southern Big South, meaning more travel for Radford.
Agreed that there is no way down (or perhaps even lateral?) from the Big South. The A-Sun is making a move to attempt to keep the Big South below by taking one of the higher quality Big South programs.
However, I have to dispute your contention of the A-Sun becoming a more "southern" Big South. Recent losses include FAU, UCF, Jax St., Samford, and Troy. Essentially the Alabama and Fla. schools. Recent acquisitions include Lipscomb, Belmont, and E.Tenn St. from Tennessee, and Gardner-Webb from North Carolina. I'm not certain about your geography but this appears like a "northward" creep to me! ;)
Therefore the addition of Radford from Virginia is not that big of a reach. I'm not certain Radford's travel would be all that greater either. The Big South has three schools in South Carolina and Birgmingham Southern in Alabama. A spot in a northern division of the A-Sun could diminish travel, or be travel neutral. Not certain the distances between Radford and Tennessee schools and Radford and Carolina schools, but it can't be too much different. Granted you would lose the proximity of the North Carolina and Va. schools of the Big South.