If the core audience really loves it, they will seek the HD signals.
How often has that worked out? No one seeks out HD signals. Even if they did, all they would find is a jukebox with no personality, no compelling reason to listen, interrupted by a repetitive station ID between every song and probably commercials. Might as well go to Spotify or similar streaming service for a far better experience than that.
Moving the format to HD always seems to be nothing more than a way to placate angry listeners for a while before it's inevitably shut off.