@Leo suggests from time-to-time on his Tech Guy show that people can replace the hard drive of their iMacs to speed them up however he says it can be difficult. I would say for the average, non-technial person, yes.
But, if you have a basic understanding of hardware, I highly suggest doing this but have a second person helping you. For me, I chose to use cheap labor, i.e. my son!, and I found it a great and fun project to do with him!
This kit I bought from iFixit was this one:
https://www.ifixit.com/Store/Mac/iMac-Intel-21-5-Inch-Late-2012-2019-SSD-Upgrade-Bundle/IF173-042?o=5
That cutter worked great. I was VERY worried I would cut too deep but the cutter is made so it’s very difficult to go too deep. But do make sure you cut back and forth several times to really get the adhesive off…if you feel it “bumpy” when you roll the cutter over certain spots, it most likely means there is adhesive still left UNLESS you are near the camera…be careful around that.
The key is to have a 2nd person to help keep the glass open while you are disconnecting and reconnecting the cables going to the monitor. I found I did NOT need any suction cups as long as you are careful.
I also highly suggest not attaching the new adhesive strips until you are sure everything is working, i.e. once you have everything installed, temporary put the display back on and hold it in place with some masking tape, boot it up, and if it works, take the monitor back off and then apply the adhesive strips.
One final note…when I did boot the iMac up with the new SSD in it, it showed two separate drives…. the 128 GB SSD (the part left over from the fusion drive) and the new SSD we installed. So, I am guessing I could have gotten by just buying the tools and not replacing the HD and just attaching an external HD for storage if needed.
Moreover, when I booted up the iMac with the new SSD installed, I got a circle with a line through it, which I believe means it could not find the OS. I did clone the original fusion drive before taking my iMac apart hoping it would boot onto the new SSD (the clone) but it seems it didn’t recognize the drive as a boot up volume. So, I ended up reformatting the original SSD (the 128GB drive) using Recovery mode and reinstalling the OS on the original SSD and just using the new SSD for additional storage.
Make sure you have a separate backup of everything before you attempt this. For me, I was just refreshing the iMac to be a stand alone server so a backup wasn’t so important for me.
Hope this helps anyone who is thinking about doing this!