There's a change in the positional adjustment because he's not catching now, but an equally large part of is that his defensive metrics (at least the ones used by Fangraphs - DRS still loves him) have been just average. Whether that will continue or is just a tiny sample size thing, I have no idea, but I think he's looked great.
And yeah, I would take Gurriel over Varsho based solely on what they've done in the first 25% of the season. I don't think that should be controversial. And I would have loved to keep Gurriel when the trade happened because he would have been a great fit to platoon, but I also didn't care that much because Varsho is a much, much better player. Refusing to pull the trigger because they wouldn't give up one year of Gurriel would have been dumb. The only thing that matters in evaluating the trade is what Moreno and Varsho do, and, like you said, that will take years to know.