switch from factory XM radio to Sirius

I would like to switch our factory installed XM receiver with the Sirius SIR-GM1 receiver in our 2006 Rendezvous but Sirius says that I cannot use the xm antenna. Does anyone know of a way to modify the antenna to fit the new unit or know of an adapter to do the same thing? Also, can anyone tell me where the XM unit is and how to replace it?