As Armoredman said: you can't get there from here.
The CZ, unlike the EAA guns, wasn't designed for interchangeable barrels. (And unlike, EAA, CZ doesn't allow for interchangeable top ends. There are subtle differences between the .40 slide assembly, and the 9mm slide assembly. One might work, or it might not... They're not supposed to be swapped.)
CZ won't sell you a .40 top end, anyway. They won't sell topends -- period.
As for trying to convert an existing 9mm: the extractor probably won't work after such a conversion, the barrel is way too large in diameter (and if you bore out the slide, you'd never be able to convert it back to 9mm), the headspace would be wrong, and the breechface might have a problem with the larger .40 rim.
And as you seem to already know, the BD (decocker) version, isn't available in .40. If you want a decocker CZ in .40, your only option at the moment is the CZ-40P, which is a limited run gun -- but very reasonably priced.
It has the 40B top end (.40 s&w) and the P-01 frame, tweaked to work with the 40B slide; it offers a decocker. Its a bit of a "franken-gun," with a few detractors and a lot of enthusiasts.
The new 75B Compact is similar (i.e., a .40 slide on a P-01 frame), and cut form whole cloth (i.e, not thrown together from to different gun lines), but it does not have the decocker -- it is "safety" only.
The 85B and 85 Combat are NOT available in .40.
(Bar-Sto gets around all of this by creating barrels for smaller calibers to fit .40s or .45s. I have several friends with SIGs in .40 who also have a 9mm barrel. Bar-Sto tweaks their barrels so that they'll work.
EAA, on their small frame semi-autos, builds them that way from the git-go; the 9mm and .40 barrels are functionally interchangeable, as are -- generally -- the mags.)
