Those are new games entirely, not just updates or expansions..The difference is when The Taken King comes out, we'll still have possession of everything we had prior to The Taken King, while when you move over to Halo 2 from 3 you never had anything from the previous one with you in the first place. You're on an entirely new slate. Honestly, WoW Expansions would've been a better comparison. When you buy the pack, you still have ownership of everything you had BEFORE the content was released, but will have to get rid of said items in order to keep up with the higher-leveled content through getting the new items which are obviously more powerful than what you had before.