Frankly, I would not upgrade this system as it has a faster Thunderbolt external port than what the internal HD SATA III port offers.
Lets look at the numbers:
- SATA III - 6.0 Gb/s
- Thunderbolt 1 - 10 Gb/s
- Thunderbolt 2 - 20 Gb/s
So depending on which exact model you have you might have a TB2 interface which is much faster!
Now lets look at the PCIe interface the blade SSD is currently using
- PCIe 2 Lane (AHCI) - 10 Gb/s
- PCIe 4 Lane (NVMe) - 32 Gb/s
So if you wanted a faster system using an internal drive I would invest the effort replacing the blade SSD with a much bigger SSD unit. Apple offers a 1 TB bade SSD but it will be expensive! If you have the older PCIe 2 Lane (AHCI) model there are some 3rd party options coming out this Spring so I would wait. So far there is no PCIe 4 Lane (NVMe) option other than Apple. In either case Apple uses a custom SSD so you can't use the now more common M.2 blade SSD's.
If you really want to replace your current HD you have some issues as you will need a special cable harness to replace the HD's thermal sensor. This is what you'll need: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later and you'll need this frame: OWC 2.5" to 3.5" Drive Adapter Bracket to hold your SSD.
Opening up the 'Thin Series' iMac's is a big under taking! Make sure you follow the IFIXIT guides to the letter as you can damage the display assembly quite easily if you don't use the correct tools and technique.