I believe that there is, though I haven't read about one yet. If your old tube radio tunes the bandwidth necessary for digital reception, all you'd have to do is plant the necessary circuit in place of the radio's regular detector (or something like that). If regular broadcasting ever goes by the way-side, such a conversion kit would certainly be something to market. I know that digital television broadcasting is supposed to take up less space. I don't know about sound broadcasting. If it was significantly compressed, your radio might not be able to tune it in. Considering the huge amounts of space the current digital 'noise' takes up on the digital stations on my AM dial, I can't see it as being too compact. I believe that the anolog broadcasts are far more compact.
T.