Tony is correct. in that doing a top end on an existing gun is a little more work than doing one on a gun that youre building up. Not impossibly hard at all, its just that you cant modify the existing bottom while fitting the new top end.
Some gunsmiths probably might argue that theres no difference and that theres no extra work involved, but id chalk that up to attention to detail.
Lastly, when you do two top ends on one lower; things can go from "optimal and perfect", to ," its in-spec, it will run". -unless you want to swap out the ejector, mainspring, heck, mag catch, etc...
I had a setup like this, and Tony does now too. If youre going to have a top end and some 9mm mags lying around in your safe, you might as well have a bottom on it, so you can just grab it and go.