My preferred pistol, cartridge combination is the P229 in .357SIG. Since SIG has stopped making the P229 in .40/.357SIG, I have heard from a few people (who have questionable firearms knowledge) that a 9mm P229 can be converted to .40 or .357SIG with just a barrel and magazine swap (direct drop in , no milling). I could have sworn, somewhere on this forum, I read that it could be done, but that milling and fitment was required. I tried searching for the thread where I thought I read this, but I couldn't find it.
My current P229 has over 30,000rnds through it. It would be great if I could buy a 9mm P229, drop in a barrel, and continue to carry .357SIG. I have been trying out a FNX45 and have been pleased with carrying it, so far, but my P229 definitely conceals better, and I believe the projectile geometry of the .357SIG at velocity is a better option all around than the 160gr Barnes cartridges from Buffalo Bore I'm carrying in my FNX. I'm using the Buffalo Bore 125gr@1425fps or Speer GD 125gr for the .357SIG.
I apologize if this has been addressed and answered. I swore I read something about this somewhere, I just couldn't find it.

If I understand correctly, it sounds like you want to go from .357SIG to 9mm and not vice versa.

Bar-sto makes a conversion barrel that allows you to do this. Per their description you will also need a 9mm magazine and maybe a lighter recoil spring:

BAR-STO Sig P-229 Conversion Barrel

I believe you'll need a .357/.40 slide. The barrel diameters are different. Or get an aftermarket conversion barrel. I'm not sure if you'll need a different locking block.

9mm to .357

.

The differences between a 357/40 P229 and the original* 9mm version:

The 357/40 slide is heavier and a less powerful recoil spring might be necessary for reliable functioning if a 9mm conversion barrel is used with the slide.

The 357/40 magazine well is wider and although a 9mm magazine will probably lock into place, it will be loose and that might—might—affect reliability.

A factory 9mm P229 barrel will not work in a 357/40 slide because it is smaller in outside diameter. Plus the barrel hood extension of the 9mm barrel is narrower (as I recall).

Putting a complete 9mm P229 slide assembly on a 357/40 frame won’t work because the locking insert is different. As I recall, though, 9mm conversion barrels made to work with 357/40 pistols usually are configured to work with the 357/40 locking insert—but I’m not certain.

I seem to remember that ejectors for 9mm and 357/40 P229s are different, but I’ve never heard of that’s ever being a problem.

I know of no way to convert a 9mm P229 to 357 except to use a complete slide assembly, and then there would/might be problems with the magazines because a 357/40 mag won’t fit into the magwell of the 9mm. That might not be true if your 9mm was a P229-1 because the magazine well is wider and will accept the wider 357/40 mags. But although I don’t know for certain, I suspect that because the P229-1 probably uses the original style 9mm barrels, the locking insert would probably have to be changed, and assuming that would be possible.

*Some of these differences were changed with the 9mm P229-1.

Added: In rereading your post, I probably didn't answer your question directly, but I assume it's more or less clear.

You can go from 40 to 9mm, but not the other way.