@Trucker pretty much has the right of it, I think. I suspect that at the very least you'd have to replace the entire slide assembly, and probably the trigger assembly as well. With the rest of the gun being mostly plastic, and the parts to build a gun costing more than the finished product, it would probably be cheaper to just buy an LC9s even without trading the LC9 in on it.
You might call Ruger and ask about it. I know that officially they would want nothing to do with such an idea, but stranger things have happened. I have read credible reports of Ruger swapping different length barrels, refinishing, even changing caliber free of charge. If the tech who picks up the phone when you call has been wondering about such a thing himself he might make it happen for you.