You should be able to interchange barrels in the P229. It is especially easy changing between .40 cal & .357 Sig. 9MM may be a little bit more of a problem if this is the caliber you purchased your handgun in since you wont be able to use the magazines for .40 & .357 then. If you start with a .40 or .357 caliber P229, you can also use the magazines for 9MM rounds (it "magically" becomes a hi-cap).
This is also true for glocks and other common .40-.357sig caliber pistols.
Barstow is just one of the better barrel manufacturers. If you don't need such quality, check in Brownells & other places for drop-in pistol barrels starting from $89-120.