You can order a conversion drop-in barrel from BarSto and you do not have to change anything as long as you are going from .40 to 9mm. You cannot go the other way.
No slide or spring changes necessary. They make it fit and work.
All you need are 9mm magazines.
My son bought me one for Christmas and I'm waiting for my magazines to try it out. Right now that is the hardest thing to find. Finally got mine from Top Gun Supply.
Check out http://www.barsto.com/category_main.cfm?ID=229&cco=68