Our favorite movie monsters all have iconic appearances. When you say "hockey mask" you think Jason Voorhees. When you say "burn victim" you think Freddy Krueger. Despite these constants, these masked men change subtly over the years. Different directors, different actors (with the exception of Robert Englund), and advances in technology all contribute to changes that you may not notice - until we line them all up for you.
Nightmare On Elm Street
Other than getting a little more grotesque over the years, Freddy's burns haven't changed much. Until, of course, we get to that heinous remake, which sees Freddy's burns as meltier and, perhaps, more "realistic."
Michael Myers's empty, expressionless mask is a visual representation of the blank, emotionless soul of the psychopathic killer. In actuality, the mask came out of simple economics: the props department on the original Halloween had to get the cheapest mask they could find. They then painted it white, teased out the hair, and reshaped the eye holes to obscure the mask's likeness: William Shatner. There are no major changes to the mask throughout the years. It changes its shape a bit, depending on whose head is in it, and Rob Zombie's two Halloweens have dirtier masks.
Since Michael Myers didn't appear in Halloween III (in a misguided attempt to make Halloween a franchise without a central villain), I included one of the cursed Halloween masks from the flick. Now that's change you can see!
Friday the 13th
It's interesting how the simplest, most iconic look can have so many changes over the years. Jason wasn't even in the first Friday the 13th film (outside of Alice's nightmare) and in the second film he didn't have his hockey mask - it was just a sack over his head. He gets his mask (and a good 50 pounds) in movie three. The mask gets damaged, it gets dirty, but it doesn't change much over 30 years of films. The two most notable changes: the mask in Part 5 didn't have a red triangle on it (continuity error?) and in Part 10, Jason got a spiffy new metal mask (he's in space; he's got to look futuristic).
Texas Chainsaw Massacre
Leatherface's mask should always be changing; after all, it is supposed to be made out of the faces of his most recent victims. I hear that in the new 3D Texas Chainsaw Massacre, due out later this year, has Leatherface in multiple masks throughout the same movie.