Assuming you didn't buy one of the very first GPWiz49s, that is. The firstgen products did not support DRS mode switching via software, only via hardware.
If you're not sure if you have the firstgen product, any bought after April, 2005 should have the newer firmware with autoswitching enabled. Randy used to offer an upgrade chip for anybody with the old ones, and I imagine he still does, but you'd have to check with him to be sure.