As evidenced in the videos past the break, this isn't merely a texture swap, as goodoldganon also incorporated Roy's moves into the game. By delegating the hack to a certain color set of Marth's (here it's the red suit), he can pick Roy at will and seamlessly integrate him into the game. Sure, the bottom of the screen still says Marth, but, for all intents and purposes, this is the Roy you remember.
It's impressive stuff, to say the least, so head past the break and check out the footage of a fighter's return to the arena.
The initial beta video
The improved version of "Roy" in action and how to select him