As has been stated above, you can just drop in an 9mm barrel and mag and you're good to go. My wife has been using the 9 conversion in her M&P40c most of this year with no issues. I took my son in law out for the first time last weekend with a 9mm setup in my 40 FS. Between the two of us we shot about 150 rounds, again, with no issues.
Midway has mags. If you are going to do this I'd get a couple of mags while you can.
http://www.midwayusa.com/product/862...lue?cm_vc=S014