replace fusion drive with ssd

+1 to an external SSD: How to Setup and Use an External SSD as y… - Apple Community. It has no risk to install since it's connected via USB, with almost identical performance.

I've been running my iMac (21.5-inch, Late 2015) from an external SSD for over a year now and have had no issues.

Jack

What year? If the screen is glued on the task can be quite daunting. And it's not recommended a normal user attempt it.

I setup an external SSD for my wire's 2015 27" with fusion drive and performance increased substantially.

You'll need to wait for someone else to answer about the particulars of the drive linked to the logic board. But then, they all are linked someway or they'd be of no use. Once we salvaged the fusion drive's data, it was essentially ignored. Removed from Finder sidebar and forgotten about.

Given the performance gain of the external SSD, ignoring it was a no brainer. External HDD storage is cheap and plentiful; no need to use the internal drive for anything.

Replacing either drive is difficult as the iMac must be disassembled, as you're aware. Once inside the machine though, the SSD and the HD of the iMac are simply connected to the logic board with cable or slot.

The "fusion" part of the process is all done in logic by the macOS... magically! 😉

When you physically replace either drive, the fusion of the two is split. A split fusion drive can be fixed using Terminal app, but it's a chore. Once the fusion is split, either drive may be accessed by the OS as an independent drive that you can erase, format and reload as you wish. Of course, you will need a bootable macOS installer or a bootable clone of a working macOS drive that you can use to reinstall the OS on your new drive.

I agree with Jack-19 that it's easier to simply use an external SSD as the startup drive for the iMac, and ignore the internal fusion drive setup.

How do you know the SSD is worn out? Usually the hard drive will begin to fail before the SSD. Even if the SSD's health attributes indicate the SSD as "worn out", it may not be an issue depending on what health attributes are involved.

The SSD on a 2015+ iMac will be a blade type SSD and any replacement SSD will likely be an NVMe based SSD which will require a minimum of macOS 10.13+ to be used (In fact macOS 10.13+ must already have been installed at some point in the past so that the system firmware is able to recognize an NVMe SSD). You should definitely consult the OWC upgrade installation videos for that iMac in order to judge what is involved in replacing the physical drive(s). The videos will make it seem easy, but there are several very difficult to see & access connectors which are very fragile and easily damaged. Plus it is very easy to crack the LCD Panel if it is not handled properly because the panel is extremely heavy and can crack under its own weight.

As @D.I. Johnson mentions, the Fusion Drive magic is all done through macOS behind the scenes.