In order for the face/on to be believable, there could be no gnarly scars, you'd have to retain full use of all muscles, and presumably your whole head going into sepsis would also ruin the illusion. So the movie exists in a reality where a major tissue transplant can happen between people with different blood types, doctors can reconstruct even the finest nerve endings, and all procedures are utterly free of infection.
Minus any STDs these guys already had, of course.
How it should have changed the world:
Being able to perform a tissue transplant across blood types would be the medical breakthrough of the century. You wouldn't be able to hear the rest of the movie over the constant cheering of formerly terminally-ill patients. Sure, in the movie they use this top-secret procedure to stop a bomb from going off. But that's really burying the lede in comparison to the millions that would be saved if they didn't waste time creating a Cagevolta Frankenstein.
And then destroying a Cagevolta Frankenstein.