It’s funny, I never thought about this change as Darth Vader learning about Luke, and it’s the first time I hear the theory about him hiding what he knows to the Emperor. I always took it as Vader and Palpatine discussing the issue for the first time, like they haven’t talked in a while and Vader is like “so I learned about this Skywalker kid, is he really my son? How is that possible?”, asking for confirmation to the Emperor. I don’t know if I’m clear.
I’m no SE defender, I don’t like this change either. But I always took the dialogue this way, it’s only on the Internet when I saw people complaining about this change that I realized maybe the intent was for Vader to learn Luke’s identity in this scene.
As for the explanation of why this change was made, I think SparkySywer is right, Lucas wanted to adress the fact that Vader is supposed to believe his kid is dead in ROTS. Or he just wanted to add some pointless foreshadowing to the “I am your father” reveal with the “search your feelings” line.