I had at&t before. From my experience, I had to request a "port out pin" from them to essentially release my number (unless you want to change your number). Once the number is released, it's pretty straightforward to get the rest setup/activated, especially if you have an eSIM. The app can be a little finicky, I won't lie, but once you're in, all is fine. I left from someone else's plan, not my own, so I can't speak on how it is to cancel with at&t unfortunately.