The SSD in the retina is upgradable but doesn't use standard parts. You have to have a drive made specifically for the retina model. I upgraded my retina to the 512GB (480GB after 7% OP) SSD from OWC.
It is the same type of thing with the MBA...proprietary SSDs, but they are removable from their slots. OWC also provides an external exclosure for the old retina SSD should you choose that option.
As mentioned by the previous poster, the RAM in the retina is not upgradable, it's soldered to the board.