Changing substrate.

If you can just vacuum the PFS out with a gravel vac, that'd be the easiest. But, I suspect that the hose/vac would stop when the sand fills it up. Not sure, I've never tried. If I'm right, that puts you scooping out the sand by hand, which I think would be kinda stressful. So you'd have to go real slow. Then putting the new gravel in. I think if it were me, and I were worried about stressing the fish, I'd probably net him out and put him in a 5 gallon bucket of water for 10-15 minutes while I scoop most of the sand out and dump in the new gravel. But, I'm totally guessing. I just added sand to tank with fish in it, and I just rinsed it and dumped it in. The fish didn't seem to mind much at all, but they're bigger, older, and used to me doing stuff like that to them.

Just one fish, I'd remove him from the tank, put him in a clean bucket with tank water and cover it with an opaque lid. Then swap substrates quickly. I just don't see the alternatives (however one removes sand and replaces with gravel) as less stressful. You still have hands and objects in his living space doing a lot of things that might seem dangerous.

If this was a larger fish, especially one that was comfortable as a petted fish or hand fed fish, I wouldn't remove him as he would likely not feel threatened. Smaller and skittish is altogether different to me.

But I'm not a fish, so keep that in mind.