How would a game developer go about giving a game an HD update like they did for God of War I and II?

Considering what goes into game development, would the devs have to remake all the textures? Or are they already high res but lose their clarity in favor of compression? And if this is the case, do they just uncompress all the textures and "reprint" the game? Thanks!|||I suggest you go to the game site of game online !