The canon reason is Cortana used nanobots on the FUD to upgrade his suit. The real reason is the new art direction that 343i took.
Maybe because 343i is too lazy to make a Hi-res original MKVI suit. That is what Halopedia says.If that's the canon reason, then why are the spartans wearing the armor in the flashback during the prologue? I believe that 343 is trying to pull wool over our eyes and make us believe that this is the armor that he's always worn, all the way back before Halo: CE. They kind of wrote themselves into a corner with this, and ticked a lot of people off.
Maybe because 343i is too lazy to make a Hi-res original MKVI suit. That is what Halopedia says.
Do you have a link to that? I'd be interested in reading up on that.
Just watched the video listed in the sources for the nanobots explanation (source #28), from the mouth of Frank O'Connor himself.
Still wish they would have shown the suits correctly in the prologue, but I can live with it.
As for the battle damage on his chest, didn't he basically fall into Requiem's gravity well EVA? Maybe its from that. That's just what I think anyway.