I don't kn ow if Vader changed his mind when the Emperor found out about the Apprentice or even what made him do so. But I'm fairy sure that he initially saw the potential on Galen to help him bring down the Emperor. I won't use the logic used on the previousa posts, instead, I'd ask you people to refer to Vector 5 and 6 (Dark Times) for a better understanding.
On those volumes, Vader seeks the cryo-stasis pod that beared Celeste Morne, who weared the Muur talisman on her turn. Vader didn't strike her down, even when he held her on his grasp and when Celeste makes her counter-attack, spawning the Rakghouls, Vader is forced to retreat thinking he wouldn't be able to take her as an apprentice.
For me, it's obvious that Galen was the long wanted apprentice that Vader was looking for to defeat Palpatine.