The amulet most likely has something to do with it. It increases his strength and everything else, but he ages faster.

maybe he just got a haircut? I think it looks kool.

Wasn't Sub-Zero's hair icing in Deadly Alliance? That could be a reason why he shaved it off, I can't explain the pony tail though.

His hair style might have changed because he does not wan't to be recognized as the man that was once marked for death by the Lin Kuei. With new power he might as well have a new look.

You know Zub might of said " Hey i think it time for a hair cut " and just got a hair cut maybe it the thing for old dudes.

if you notice in the screenshot of the chess game it shows subby wearing the medalion.. in the gameplay shots we see subby wearing 2 different outfits with no scar on his face and no medallion... and his costume looks a LOT more like scorpion's MK:DA outfit with those pads that have what looks to be snake faces on them (whereas scorpion's pads have skulls on them) which leads me to believe its the dead sub-zero from MK1; its logical because of the reasons i pointed out and the fact that they mentioned every character ever to be in a mortal kombat game will be in MK:D (at least in konquest)

or maybe frost cutted his hair, thinking that his strenght was there.lol

Why have the females boobs gotten much bigger over the years or why has Jax's physique become so massive? It's because they wanted a fresh look.

..........or maybe just the medalion........