Nothing at all wrong with de-priming live primers.
Unless the cheap 9mm is Berdan primed, which you can't de-prime.
Otherwise it is perfectly safe, and done all the time by experienced reloaders.
I agree it would be possible to make .38 Spl loads out of pulled 9mm, if you just had too.
But the result would not be ideal in a .38 Spl.
The recycled primer would work just fine.
As would most 9mm powder charges dumped straight into a .38 Spl case with the same 9mm bullet weight & size used in it.
But why not just keep a 9mm pistol around if you have all that extra 9mm ammo to fool around with?
If you're in engineering, not so much.
9mm/.357 convertables work just fine.
As do .45 Colt. .45 ACP convertables.
A 9mm fired in a 9mm chambered cylinder runs 35,000 PSI pressure.
Kicking a .002" undersize bullet in the butt with that much pressure upsets or swells it to fit the .357 bore.
A .38 Spl. however, is running less then half that much pressure, and hitting a Jacketed 9mm bullet in the butt with half a kick won't make it much bigger, if any.
.45 Colt, and .45 ACP use the same .452" size bullets to start with.
rc