When you're cast on Game of Thrones there's no telling how long you'll be around. The constant threat of your character being killed off is one thing, but the threat of being recast is just as bad. While franchises like Harry Potter clung to their actors year after year, Thrones has shown no qualms about discarding actors in favor of exciting replacements. Just ask the 500 or so guys who played the Mountain.
When the show announced nine new additions at Comic-Con (leaving out a very important book character, mind you), they announced that the role of Princess Myrcella, Joffrey and Tommen's little sister who was shipped off to Dorne in season 2, had been recast. Aimee Richardson, who used to play Myrcella, is taking the news in stride. In fact, her reaction is pretty much perfect.