Unfortunately "NO" it would not be a straight swap. You would need at minimum the 9mm barrel and the slide with it as well. You can use the same return spring assembly. The USP 9 and USP 40 share the same lower frame and its pretty much a straight forward swap. Check out my thread on the swap I did:
As IronChef said, the slides are different. Dimentionally they are the same, but the breach face on the 9mm is made for 9mm rounds, and the .40 is made for .40 rounds (IE, its wider) as well as the extractors are different (again, 9mm made for 9mm and .40 for .40)
Yes, you need a complete 9mm upper assembly...OR you can get a 9mm conversion barrel from EFK. Admittedly I only tried mine for the first time yesterday, but it did work. The accuracy was better than I had expected, and there didn't seem to be any trouble seating or ejecting the 9mm brass despite the larger breach face. I did have one FTF (Failure To Feed), but I was using an old .40sw magazine. I am going to try again with a brand new 9mm mag and see how that works out.
FYI - I am NOT recommending that you get a 9mm conversion barrel for anything other than range use. I just like the flexibility for cheap practice $ and lighter recoil for shooting with beginners.
Also, I plan on giving the conversion a complete review after further evaluation. Hope this helps.
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