ch: So it would only set a precedent if the Republicans wanted to defund a law before it came into effect rather than if they wanted to cut entitlement spending for a law that is already in effect?

No what sets the precedent is the Republicans wanting to overturn a law that's been passed, and then trying to set a precedent that would better enable them to attack the funding of laws they dislike.

ch: I'm not seeing the major difference that would cause the two situations to be different.

You're not seeing what you don't want to see. Nothing I can do about that.

Most Americans, fortunately, can see the difference.