HI Gary,
The best answer is: Use SHOUTcast v2 as this have mutliple mountpoints (IceCast have them as well) so you can set different bitrates for same stream (they will be available under the same port!)
If you really wan't to use SC 1 you can try to set sc_trans to capture stream from main server (set playlist with url in it) and re-encode it to desired bitrate but this will be having different port for each bitrate and it needs some more advanced SHOUTcast / server knowledge.
Regards