I ran into this with my '01 Grand Prix, so I called Onstar and had them check and my '01 was analog only, and could not be upgraded. I think around '02-'03 they switched to digital ready, and '04 became digital only. I would think anything '03 or newer is upgradable, anything '01 and older I know is SOL. I don't drive my GP that much so I dropped the service and yanked all the onstar crap out (had 2-3 different adapters & filters to use with my aftermarket HU-what a waste that was).

Mine had the Digital Onstar but I know the 2003 Bonnevilles dont have it. Btw my car is a 05 not a 04 as the dmv just sent me a letter.