The Ram and CPU are off the shelf parts, you could go and buy them before the nMP hit the shops.
TB2.0 isn't new either, the late 2013 Retina MacBook Pro had it before the nMP.
I've not hear of anyone trying a SSD swap between a nMP and 2013 Retina MacBook Pro, but judging by the fact that Apple don't charge more for one than the other, there shouldn't be a lot of difference. As for why upgrades are hard to find, it's hard to image any situation where you wouldn't be better off adding more SSD space externally rather than internally, which means throwing away at least 256Gb of SSD.