I can't see a reason CBS would want to spend money on 107.9. In SF, there was a plan that involved moving a highly rated news format to an FM simulcast. In Philadelphia, the same thing is at least a possibility.
For CBS, there would need to be a reason to purchase that either involved a format hole or moving/simulcasting a reasonably successful existing format. WHFS-AM isn't one. "WHFS" on 97.5 isn't setting the world on fire, and won't either, though it is nice to see it back.
1580 is a Washington area station, 107.9 is less of one because of 107.7 in Manassas. The old HFS on a better signal than 97.5 might be a reasonable proposition, but at what price? And with 107.9, you're looking at a station whose signal is as much like WRNR as WLZL.
Why move WJZ-FM at all? It reaches greater Baltimore. That is the target for the programming.