Calibre changes on Sigs

Question
Is it true that one can change the calibre on certain Sig-Sauer pistols? If so, what models can be modified, and how complicated is the job to do? While we're on the subject, is it a good idea? It sounds like it would be nice to have 2 guns for the price of one and a half, but are there drawbacks?
Thanks for your time and attention.

Answer
Ed,

When SIG came out with the .357 SIG caliber, they were making available .40 S&W barrels.  Because the .357 SIG is essentially a .40 necked down to a 9mm, nothing had to be changed but the barrel (magazines, springs, extractors, ejectors - all remain the same).

Technically, there is no down side to this.  Practically, I myself have never understood the .357 SIG cartridge, but if you had use for it, this is about the cheapest way to get two guns for the price of one (+ the barrel, which you can get for less expensive if you get it from after market barrel mfgs like KKM, BarSto, or others).