The change from Halo:CE to Halo 2 was huge because it was an entirely new suit of armor (Mark VI) thus explaining things like the lack of a health meter. Halo 2 to Halo 3 was about the same but with an increased polygon count to make the armor more detailed on the Xbox 360 but pretty much the exact same look.
343i said there would be an explanation as to why Chief went into the pod looking like himself from Halo 2/3 and came out looking so different. I mean the helmet is the only thing recognizable from the old armor and even that has changed a bit. It better be a good one.
The Spartan IV's are wearing Mark VII armor and that is going to look very different from anything before. I you are worried about the look so much, keep in mind that a ton of it will be customizable like in Reach. All of this has been confirmed by 343i in the last week of so.
If they use nano technology to explain chiefs new look I will be disappointed. That's like the midiclorians in Star Wars. They cant just make crap up to keep the story from falling apart. Oh and that first trailer wasn't in-game and might not even be played out that way. Would be great if they show us the opening to the game at E3 with chief wearing his Halo 3 armor out of the pod and getting new armor or upgrading it later on.