Yes. You can upgrade to a newer MacBook Pro by just swapping the SSD. I looked around the webs and couldn't find whether it was feasible, so I just did it.
I had a Late-2013 MBP and a Mid-2015 MBP. Both are same in every aspect, except (1) GPU, (2) TrackPad, (3) Keyboard, (4) Battery.
- New AMD Radeon R9 M370X vs. older Nvidia GeForce GT 750M. Both discreet GPU performs well. Nvidia has a faster clock but AMD has slightly better benchmarks.
- New TrackPad had Force Touch. This means I didn't need to keep two fingers to press; one to aim, one to click the bottom.
- New Keyboard feels more responsive, more rubbery, less force needed to push. This slight difference can only be felt when gotten used to the older one.
- New Battery is a big issue for me. I had close to 500 cycles on the old laptop, and this new one has like 20.
I opened both and swapped their SSDs. To open Apple's proprietary screws require a Pentalobe P5 (1.2mm), and a Torx T5 (or was it T4?).
Both booted as normal. Not much difference in hardware. I did it for science and curiosity. Plus I could air blast the dust out of the vents.
I was signed out of iCloud and other email accounts. Safari and Chrome extensions were lost. Safari extension LastPass was already logged in when I reinstalled, and Chrome too.