Just as valid as Xph... Xiph... Xsomethingorother's post. Only he gave reason. I do kind of agree with Slimmy, Halo 1 and 2 engine wise were near carbon copy, Halo 3 gave a few modifications to the engine (I mean let's face it, the engine worked amazingly anyway). I cant comment on the other games though because my interests just never really grew.
Actually, Halo 2's engine was build from the ground up. There wasn't anything that was copied straight from Halo 1. For Halo 3, changes where made to accommodate the jump the the Xbox 360 though it was still based off of Halo 2's engine. From 3 to ODST, the engine remained unchanged for the most part, with the exception of the ability to do full screen effects (such as for VISR mode) and other small tweaks.
I'm not sure about the engine for Reach but it seems to me it would have the same scale of changes as from 2 to 3.
As for Slimmy's comments, I'll admit, Halo's fundamental game play hasn't changed much but that's what I like about it. What keeps me on for the next game is the additions they put in. For example, duel wielding in H2, equipment in H3, and armor abilities and game types for Reach. It's changed it just enough to make it fresh and interesting.