I took the plunge and bought a Samsung 850 EVO 500GB SSD to get some more storage space in my iMac. (when I bought the iMac I configured it with a 250gb SSD only) After some research I figured out that I had to buy a SATA cable as there is none pre-installed cable with my configuration. Nor does it come with a HDD cradle/bracket, but I solved that with two strips of super sticky double sided tape to mount the drive.
When booted up and logged in a message popped up telling me that it had found the new SSD drive and wanted to format it. So I formatted it and it works great.
Oh, and if you have one of those very thin plastic "credit card/business card" you will be just fine using that to separate the display from the body. No need for a guitar plectrum or the "pizza cutter" shown in this tutorial.
I have the same question.