I would strongly recommend you take the system back and buy the SSD model straight up. Trust me its the smarter choice!
If you can’t then you’ve giving up access to the faster PCIe/NVMe blade interface the SSD or Fusion Drive models have. I would go with a 512 GB model as its the ideal size when you match it up with a better HDD than what Apple installs like a Seagate SSHD drive.
Independent dual drive configs are better than Fusion Drive’s
OK, can’t return it then you only have one option in swapping out your HDD for a SSD. Do keep in mind you’ve loose your warranty service doing this and if you’ve never opened one of this iMac’s there is a good chance you could damage something in the process (people often damage the display). So in the end you don’t save anything!
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