Windows 10 Boot Camp on Mac mini Fusion Drive is running very slow. How to replace HDD / upgrade disk to SSD without reinstalling OS?
I backed up all my data first so I could try a couple of methods before I found the right one described below.


I carefully replaced the original internal Apple HDD with the same capacity ADATA SU800 1TB SSD. Lack of a special T6 Security Torx Screwdriver was a little problem but I luckily could brake out security pins from the chassis screws, making all of them a regular torx (though I strongly recommend buying a proper screwdriver on eBay as well as the logic board removal tool instead of my laundry wire hanger, especially if your Mac is still covered by a warranty). A detailed instruction of disassembling a Mac mini is available on iFixit.
When the SSD was already inside (I didn't touch a PCIe disk at all), I attached the old HDD as an external drive through a USB enclosure and booted up from an external optical drive with an Acronis True Image HD bootable DVD (you can try another such third party software but I found that cloning a Boot Camp Partition with the Apple Disk Utility doesn't work well).
Then I cloned the whole HDD with the "as is" option to the new SSD without automatic resizing! (SSD was a little bit bigger so I tried that but the result of such attempt was a Fusion Drive malfunction, no access to a Recovery Partition and impossibility to update macOS because of an error). Still don't try any action with a PCIe partition! 
After complete operation, disconnection of external drives and restart, the mini instantly started up exactly the same as before the disk replacement. Both operating systems are running properly but many, many times faster.
Finally, just in case boot in a macOS Recovery Mode and perform a Disk Repair of a Fusion Drive and a Macintosh HD partition as well. Excluding one possible warning about some small capacity mismatch, all operations should finish successfully and you should be able to update macOS to the latest version without error.