I assume you are working with a vendor such as OWC or iFixIt to get a kit of case opening tools, new seals, and parts you need to crack open your Mac and perform surgery... Swapping out the hard drive or fusion drive or existing SSD requires cracking open the case very carefully and using a "pizza wheel" or guitar pick-like tool to split the foam tape. Then there are quite a number of tiny, fragile connectors to dislodge (camera, antennae, monitor, etc.). Then you get to disassemble the innards. OWC has step-by-step videos for nearly every iMac model. iFixIt has Dozuki guides for the same disassemblies. I used both when swapping out memory and drive in my Late 2013 21.5" iMac.
Yes, while in there, I would max-out everything. Maximize RAM. Change the HDD for an SSD. The fusion drive, on models that have it, is a virtual drive created from a small SSD and a regular hard disk. So your iMac should have a HDD AND an SSD module. I believe you can replace both, but the folks at OWC can answer with certainty.
During disassembly, be sure you have split all the foam tape, and go very slowly. It is extremely easy to *break* the monitor screen during removal! It is easy to damage the camera at the top of the screen, so work gently around that. The wires to the camera and the screen are not very long, so don't lift off the monitor until you reach in and disconnect them. If you need to remove the power supply, be very careful to watch the video or read the Dozuki guide, because it is quite tricky to get in and out.
It took me about three hours to perform the surgery on my iMac. It was definitely harrowing, but worth it! The machine is like a rocket, compared to performance with just a hard drive.
The one thing you might want to do FIRST is to configure the new SSD before proceeding. My kit came with a housing for my old hard drive. I put the SSD in that, formatted it with Disk Utility as an APFS volume, installed a virgin copy of MacOS, and installed virgin copies of all my software. Then I used Mac Migration Assistant to migrate my user profiles and data (We have five users of one computer, each with a different account).
After all that, I did the installation. That way, the computer booted up normally on the *second* try (I left the screen loose for the first two tries, and was glad I did, because the monitor cable was not secure!). Once you test the machine and know all is well, use the seals from your kit to re-glue the monitor to the base frame of the iMac.
The great folks at Other World Computing can help you with the project. They have helped tens of thousands of us with this.
https://eshop.macsales.comWATCH THE OWC PROCEDURE VIDEO for your model before ordering anything. You'll be glad you did!
I assume you are working with a vendor such as OWC... (