Upgraded my iMac 27 inch Retina Late 2015 with Samsung SSD
I was looking for better speed from iMac by the addition of an SSD and more RAM.
The upgrade was pretty straight forward following the iFixit and youtube examples. I bought the iFixit adheasive tape kit with pizza cutter, spudger, plastic credit cards, heat sensor for the iMac (OCW) and a mounting bracket 2.5 to 3.5 (local computer store). All of the tools made this upgrade very easy and are highly recommended. I didn't get the foam support for the computer when you remove the screen but used a small box to get the same result. I didn't find it as difficult as the guide suggested but you must really take your time when breaking the adheasive seal and thoroughly go around the edges with the pizza tool. I recommend setting aside a full day with no distractions to accomplish this task.
The adhesive is the trickiest part of the whole affair. It is fairly time consuming to remove the old adhesive from both the screen and the mounting area so that you can apply the new adhesive. The bottom area of the mounting surface may need a scrapping tool to remove the rest of the adheasive. The spudger worked ok for removing this but you may need something a little harder. Don't use liquid adheasive remover as most could damage the screen or the board, use your nails. For the bottom adheasive strip the guide says to use a sharp tool to cut the bottom adheasive, DO NOT DO THIS, as there are pull tabs to the right and left of the screen on the late 27 inch iMac to make the process to remove this much easier. If you cut the strip you will spend more time removing debris from the screen and mounting bracket. When reassembling put the adheasive tape on the mounting area and the back of the glass (14,16), and using the non-numbered tabs and the spudger to guide the tape with the holes for alignment. Try to touch the tape as little as possible, use the spudger for alignment and guide the tape into place as you remove the non-numbered side of the adheasive tape, take your time. Do not remove the second side of the adheasive tape with the numbers until you are ready to reattach the screen. I reconnected the screen, installed the OS then reattached the screen with the tape to make sure everything worked as expected. You can use some electrical tape to hold down the drive cable inside the case, as the 2.5 inch drive and the adapter allows for more wiggle room of the cable and I didn't want it to vibrate. The glass did slip off when I was removing the second side of the mounting tape, I recommend having someone hold the glass when removing the second side of mounting tape. Nothing was damaged but I had quite a scare as it slid down 6 inches and hit the mat with a thud. Oh yeah, have a soft surface or mat present when you are doing this work.