Look at my above post. He gets bulkier and his skin color changes from brownish to redish/black. He looks much more athletic at first, and then activates his body armour when he's about to fight, look at the shoulders.
Like goosebumps, but with armoured skin.
It seems to move in some way, on the shoulders especially, like it was changing shape or something
It has some kind of metamorphosing and changing of texture on the skin. Or he is growing armor (like Johnny Handsome said), making his skin get thicker, or he actually can camouflage himself with his skin like a real chameleon. Look closely on the skin of Upgrade at 00:17.
Thanks for lighten me up, guys. I did not catch that kind of metamorphosis...quite a quick shot.
It seems, he and his body is preparing to fight, as you say. It's some kind of offensive/defensive biomechanism... He grows spikes, you can see it on his forehead and also on his left upper arm as well (or the latter could be only skin tone change, I don't know...now I'm already seeing little spikes
This wasn't in the script, but I can say that I also like this.