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Not quite. I updated my SSD on my mid-2012 rMBP.
I'm quite aware of that upgrade route.

Before the retinas came along, the manual stated that the hard drive and SSD were user replaceable items. They are not longer deemed as such by Apple, even though some earlier models CAN be upgraded.

I should've elaborated a bit, though.
 
There is hope that in a year or two some major PC manufacturers will start using PCIe SSDs. This day perhaps there will be 3rd-party SSD replacements for Macs with PCIe storage.
 
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.
 
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