I have long argued that virtually anything drawing a large audience on AM can draw a significantly larger one on FM.
The reason is simple: Distribution.
Check your Arbitron information and see how many 25-54 listeners never go to AM at all.
That is your opportunity cost of being an AM station. It means you are invisible to that audience. It means you don’t have “distribution” for that audience.
Moving WTOP to FM is the equivalent of opening a bunch of new Wal*Marts in areas which had not previously known Wal*Mart. It will inevitably improve distribution, and that will inevitably improve ratings.
By the way, “distribution” is one of the great advantages of radio as a medium over Satellite. And it will be a huge advantage of conventional radio over HD.