A few consoles have had minor expansions that can be added. N64, Sega 32X, etc. But that's usually just more memory or a new media to allow for more game content (but not actually "upgraded"). Actually changing the graphics capabilities would be more difficult to implement and you'd just end up with a multi-tiered systems of games and hardware that would essentially remove the main selling points of consoles. Adding those kind of features would lose more customers to PC gaming than it would gain through added features.