I remember that KWAM-FM changed their calls to KJMS around the mid '80's when the went to the "jammin" format. Perfect call letters for that format. I guess they made the move to Urban AC around the time Clear Channel bought them up in the late '90's. I never could understand why they didn't change the calls to be something more reflective of the format.
On the other hand, if I'm not mistaken, the only time they say "KJMS" is with the legal ID at the top of the hour.
But...many media folks and listeners (and folks in San Antonio at corporate) still refer to the station as "K-Jams".
My guess, for whatever it's worth, is that clear Channel wants to holds on to the call letters for use in the future at one of there other stations. This has happened before.
greg hamilton