I just wanted to show that it is possible to upgrade the SSD (and memory in fact) of a Samsung Chromebook Series 5. This particular model is from Oct 2012.
I'll show you photos of the internals and parts that I used.
So first up a photo of the old and new SSD.
Make sure that you get the part type right, SDSA5dk, otherwise the new SSD might not fit.
To prove that the upgrade worked, a screenshot of
df -h, that shows the new 56Gb volume.
Both memory strips and the SSD I got from Amazon, turned out a US based vendor had them and shipped to NL.