Can you upgrade an iMac with retina display?

Replacing the Fusion Drive with both a SSD and an harddisk

This month I did an attempt to upgrade my 27-inch iMac 5K. I wanted to replace the fusion drive with a SSD and a large hardisk. In addition I decided this was a good time to upgrade the RAM memory as well, maxing it out at 64GB. I was on a mission to create the ultimate iMac, but things didn't go to plan!