Lowering resolution on DTV TiVo? - DirecTV DVR

For Lowering resolution on DTV TiVo?, relating to DirecTV DVRs and receivers. I have a Hughes GXCEBOT DirecTV receiver with TiVo, and I am wondering if there's any way to lower the resolution so that I can record more than the standard 35 hours it holds. Any thoughts on how to do that? Is there a setting or something I can change?