The SSD on late 2013 and newer rMBP's is user replaceable you just have to replace it with another Apple original.
They are available on eBay and other places. I did it myself. Now have a 1TB on my rMBP and the 512GB from the rMBP is in my wife's MBA.
The wife's 256GB sits in an OWC external USB 3 box for backup use.