The Mane Six - and most other ponies - have more than one talent. In another world, they could have had entirely different cutie marks. That doesn't mean they're not friends.
Nightfall Sheen, prized student of Lord Lumin Draconis, has been assigned to the Longest Day festival in Dragontown, but he has more important things on his mind. Namely the threat of Lumin's corrupted brother returning from the moon.
Starswirl's unfinished spell didn't swap marks. It merged destinies with other worlds where things happened differently. In a world where it all went wrong, only ponies from the world where everything went right can help.
Luna's has enough to deal with as Princess Twilight's student. Between nightly nightmares of moon princesses and the threat of an old enemy returning from Equestria's past, Luna isn't going to be getting any sleep any time soon