Thanks, this sounds pretty much how it is in the game.
I think my problem is with understanding the swapping at level up. As I was (trying) to describe in my other comment, during level up I learned a new spell "x". Then I was trying to swap in spell "y" without "forgetting" another spell first, because the game let me do this. I thought that I gained a new spell slot so that's why I didnt have to forget another spell, but after level up, spell "y" wasn't available.