Along with the upper you’ll need a mag block(it holds the magazines in place in the magwell), a 9mm hammer and buffer change. You can get the bolt ramped then you wouldn’t need to change the hammer.
If your going with Rock River check their web site they have the other things you’ll need. There are other manufacturers who make parts, kits or guns. There is another site that has a pistol caliber AR forum with an FAQ that might be helpful to you.
BOBK48 is correct, the hammer, buffer will need to be changed as well as a mag block to accomodate the 9MM mags…use colt mags. C Products is coming out with their version, similar to colt but half the price.
If you’re thinking about frequently switching from 5.56 to 9MM you may consider getting a seperate rifle. IMO
RRA 9MM Complete Uppers will work fine. It’s a matter of preference.