First off it's been revealed that the Chief's newest armor is just the Mark VI but with FIRMWARE upgrades. Firmware can not mean the whole armor plating was changed as well.
While I was never a big fan of the traditional Mark VI, there just needs to be an actual explanation for the change in look, not ARTISTIC LICENSE.
To me this would mean that if a Halo 2 Anniversary is made, the Chief would not be wearing his famous Mark VI look but instead the new armor.
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I'm hoping that there will be an in-game cutscene or something that would provide a reason for the change from Mark 6. So, we all have to wait until Halo 4 comes out to see what kind of explanation is given as artistic licencing/cryo sleep modification is too much of a deux ex machina for the drastic change in appearance as stated by other posters and the fact that raw material is needed for such a thing to happen. That being said, I still think the new armor for Chief and multiplayer Spartans looks incredibly odd and clunky.