If you're talking about doing on-the-fly bandwidth rate conversion for some sort of a streaming mp3 server, then I too would be interested if there is anything out there that can do it!
It should theoretically be possible, but it would obviously require some extra compute resources on the server end. We'd be talking about decoding a frame, throwing away even more frequency components (downsampling is the only question here), and re-encoding. This should be easy to do in real time!