I agree that changes need to be regression tested, but assuming good software design, some of the convienience systems (a setting for a different cold weather ICE assist) could be rolled out with less testing involved than inclusion of changes to a more complex subsystem (battery management, motor control, etc.).
If GM is going to play in the high tech world of SW, they will need to provide a means to test and roll out software updates.
This is also a means for GM to generate additional revenue, so they can cover the development and testing cost. Personally, I would pay for an upgrade to include features like Hold mode or a choice of cold weather ICE engagement. I might even consider a software maintainance contract that provides updates when they become available.