Depending on how long it’s been since you’ve played, they’ve changed the rules probably like between two and five times since then, so you’d have to learn how to play pretty much from scratch anyway!
Every time a new army book is released, they reset which models are in the game, and how those models are. Some armies get drastic changes which basically makes you buy, and paint, all new models. The rules regarding how painted your minis needed to be in order to participate in official Games Workshop events changed, but if you actually paint your models it’s not really an issue.
Also, if you played a foot-heavy army in 40k, the 2" of 2 different model rules changed the way things moved significantly.
Warhammer (aka Fantasy) stopped existing, and was replaced by AoS in the 2010’s. So if you played fantasy and not AoS, it would be a huge change.
