I own a .40, the build is beefier for the .40 loads compression. To go to 9mm in a .40, all you need is a barrel & magazines. BUT, to go from 9mm to .40 you need spring, slide, barrel, and magazines.
** Note- I have read some forums from the USA that guys are using their .40/.357 magazines to load and shoot 9mm through a 229 with a 9mm barrel, but I cannot say whether or not it works. It may be Barrel dependent (I think one guy said it worked with his Barsto but not his other one)**
IF YOU TRY THIS YOU ARE TAKING YOUR OWN CHANCES
Here in Calgary, the different in price right now for shooting .40 vs 9mm is minimal currently...
250 x 9mm : $65.00
250 x .40 S&W: $72.00
Thats only $28.00 over 1000 rounds (100 magazines shot - only $0.28 a mag). So to shoot .40 it isn't THAT much more expensive.
9mm is easier to sell later on, as a lot of new shooters "want 9mm", but the .40 has a heavier slide making recoil less.
My P229R is heavy, and SUPER easy to shoot + reacquire target fast due to the low recoil. I can't imagine how much easier my 229 would shoot 9mm ! haha.