We've done this with Berettas - .40 cal barrel for 9mm barrel - and it works fine. They use the same barrel blank for both. So it got me wondering if the same could be done to the 4003TSW with a Model 5903TSW barrel (Without slide alterations of course). Bar-Sto doesn't make S&W barrels so that's not an option. The outside diameter of the 5903 barrel would have to be the same of course, the same for the barrel hood. Since the .40 breech face is larger going from .40 to 9mm it isn't an issue with the Berettas. Don't know about Smiths though.
If you've done this let me know how it works.

With the TSW series the barrel, slide and frame are made as a matched set. So the fit of the barrel would be a problem. Also the 9mm mags are wider than the 40s' and the 40 mags have a dimple on the front of them so they won't go into each others gun.

I have fitted complete 9mm slides to the 40 caliber frame.

The lug on the 9mm is larger than the 40. The 40 magazine will work with 9mm ammo but won't keep the slide open after the last round is fired. The 9mm magazines work fine in the 40 caliber frame.

I have fitted two of my pistols, 411 and a 4006, with complete 9mm slides.

I wish I could find a 357 Sig barrel that would fit the 40 slide.

Yorkie Man, were the pistols you fitted TSWs?
Since the slide won't lock, the 9mm mag must be shorter right?

In what way is the lug on the 9mm larger than the 40, longer, wider?

Thanks.

No. One is a 4006 and the other is a 411. The slide release for the 40 is the one that I use for the 9mm slide when it is installed on the 40 frame.

The lug on the bottom of the 9mm barrel is wider than the 40.

And it fits perfectly in the .40. Now to take it for some range time.