Super helpful.
I had a Stripe pool of 2x 6TB disks that I considered "expendable" if it failed (only movies, shows, etc) but then I growed my RAIDZ pool from 8TB to 6TB and was left with a spare 6TB drive - suddenly that Stripe could be a RAIDZ too...

So I offloaded most of the data to the bigger pool, created a temporary Stripe on the spare drive for the rest that did not fit.
Then after having duplicated all of the data elsewhere, I destroyed the 2x6 Stripe, created 3x6 degraded RAIDZ, copied data from the temp Stripe, replaced the sparse file with the spare and let it resilver, and finally copied the rest of the data to the now-fully healthy RAIDZ :)

I know how to do this, and why to do this, but its very very useful having the recipe outlined this clearly.

I did notice that I don't seem to be able to replace a "FILE" disk in the GUI using CORE 12U8-1 unfortunately.

The CLI works fine with a GUI imported pool though.

[code]
zpool replace testpool /root/<sparsefilename> gptid/<guid>
[/code]

Best warning ever and excellent follow-up:
If you're going to write down instructions that people should never follow, then write them in a clear and complete manner.

Wanted to say thanks for this, very useful advice for a super niche case most people probably won't (and shouldn't) really need!

In my own use-case I had an awkward juggling match to deal with; when I got my four bay box, in an ideal world I would have got four disks and just setup a RAIDz2 on day one (for half capacity with two disk redundancy and maximum read performance not a concern) but because of the disk prices at the time it wasn't within my budget to get four smaller drives, biggers ones were better value but I could only get two so had to go for a basic mirrored pair.

Fast forward to when I can afford another two and my choice was either add another mirrored pair and just accept single disk (guaranteed) redundancy, or use what your guide showed. In my case I created a RAIDz2 with two disks and two sparse files, offlined both files, copied my data into the degraded pool, then replaced the sparse files with the disks from the original mirror one at a time.

While doing it this way meant there were technically always two full copies it was still pretty hair-raising, but my offsite backup doesn't keep as many historic snapshots so I really wanted to preserve them if I could.

Excellent resource! Thank you so much! In my case, I have a backup of all data, so this is a safer way of creating the pool without going down to one copy of my data.

Excellent instructions for something most people should never, ever do - but if you're going to point a loaded gun at your own foot, this at least gives you a chance not to blow off your toes.