Nothing but the RAM is technically user upgradable.
The fusion drive uses a different style of SSD that is either proprietary to Apple, or just a less common style. For anything other than that, you have to peel the screen off, which is glued on.
The graphics card is soldered to the logic board and not on its own board as far as I have seen, but I may be wrong.
The CPU is socketed and removable, which would lead me to believe that it's upgradeable, though you never know with firmware, EFI crap and stuff with Apple.