"We can make him better than he was—we can rebuild him!"
Apple no longer markets a 17" MacBook Pro. But I'm a video editor—I need as much screen real estate as I can get, on a burnin' fast portable platform that can handle 4K (UHD) video streams. AND my original Hard Drive was only 120GB. With Avid Media Composer and Adobe Creative Cloud apps installed—there was no room for the latest OSX version! My only hope was to upgrade: A 1TB SSD for the System and a 2 TB SSD for Project Files.
Nervous at crackin' and hackin' the machine with which I make my living—especially because (competitor) OWC had told me what I was trying to do was impossible—I held my breath and dove into iFixit's procedure. iFixit's Essential Electronics Toolkit was superb: Nice assortment of magnetic tools tips, just the right effectors and widgets, and a nice professional case (for very little $$). Twenty-five minutes later, my old Mac woke up, scratched it's head for a moment—then took off like the proverbial bat out of Hades!
If replacing the System drive with an upgraded device (e.g. SSD), use SuperDuper to clone. It's nearly impossible to get a clean disk image using Apple's (updated) onboard Disk Utility. And Carbon Copy cloner was, well, so crotchety to work with, I gave up.
Also: You may read/hear that a 6Gb/s SSD will not work in the optical drive bay of a 2011 MacBook Pro (8,1 - 8,2 - 8,3). Mine does. (Possibly your mileage may vary.)
I do not know if iFixit regards OWC as a competitor. I will only say that the OWC solution was more than 2X the price, and OWC's documentation was squirrelly on this particular upgrade.