The soil is supposed to be capped anyhow, so you don't need to remove your substrate and rinse it.(many planted tank forums are referring soil to dirt, not sure if you are aware of that).
And like I said, you can remove the fish, they can remain in another container and remain heated. Once everything is settled down, the tank will be warm enough again and your fish can go back into the tank.
Yes, I know soil is dirt.
I'm referring to the substrate cap (the non-soil one that is theoretically in there now).
Unless we're crossing wires and I'm misunderstanding you, you're talking about a way to add dirt/soil to the bottom of an existing non-dirt/soil substrate (gravel, sand, etc), right?
Any substrate in an established tank is going to have all kinds of gunk trapped in it that you don't want to release without risk to the fish, even if you change the water, much will be newly-exposed and likely start producing ammonia and other junk in the water. So major disturbances to existing established substrate should be kept to a minimum. Even carefully "shimmying" in frozen blocks would be a pretty major distubance, if you're coating the whole tank bottom with them under the existing substrate (which would now be a cap for the dirt/soil).
Of course, if I'm misunderstanding what you're proposing, then perhaps this warning doesn't apply, and I'd love to hear more about this, because it does seem interesting.
(I mentioned the temperature issue because you said "Can also remove the fish if you want
" but I don't see that as particularly optional for major work like this.)