According to Delphi... so far...
I have been in contact with Delphi on this very issue since I am looking to upgrade the radio in my 2000 Grand Am (not class 2) to one of the newer MP3 playback units (Class 2 capable). In theory it looks as if all I have to do is change the dash bezel and the plug. Delphi neither confirmed nor denied this theory, but they did send me a pinout diagram. It looks like from the diagram that there is not only the class 2 power in, but also the traditional 12 volt batt and 12 volt IGN on the first 2 pins of the top row in the 24 pin connector. Class 2 is on the bottom row pins. These would be the ones next to the AUX 12 pin connector. This points out that these might be a class 2 bypass of some type for non-class 2 vehicles. I'm still getting info, but in that, it would seem that you can upgrade to a newer class 2 capable head unit in a non-class 2 vehicle, but, it's a bit harder to use a non class 2 unit in a class 2 enabled vehicle and you would need and adapter.
Anyway, that is just my theory so far. I'll let you all know how it goes.