I have an all black P239 in .357SIG and a hard chrome-looking refinished one in .40. I was thinking of swapping all the controls between the two for a reverse-accents look on each. So it'd be hammer, trigger, decocker lever, slide catch lever, takedown lever and mag catch.

Will all of these from one fit on the other?

Yes, everything is the same for 357/40. The slide catch is different for 9mm.

Great. This should be a fun project and, if I don't muck it up, cost me nothing.

The only thing or spring to mess up, well at least for me, is the trigger bar spring. I have gone through a few at times, mostly due to the install of an E2 grip. Everything else on the gun is tough as nails.

I know the trigger bar spring needs to be shaved down for some types of grips and/or installing a SRT kit. But they don't make E2 grips for a P239, AFAIK.