I just heard "Grenades don't reliably destroy cover without sapper" and that was it. However, even with multiple Vanilla games under my belt, I don't feel I had to relearn much. When you realize (or just get flat out told as I'm saying here ) that grenades solve all problems in Vanilla, you should take that as a sign of something that is nerfed in Long War.
A quick way to tell whether or not you'll have to unlearn something is how unfair it feels. While you might have to unlearn some things from Vanilla when playing Long War, you'll pick up on plenty of perfectly good tactics. You'll have a lot of fun playing through it.
Long War 2 likely has more staying power, but there's nothing at all wrong with XCOM 2.