Lucy Beale was completely destroyed because of it (doesn't matter if you think NewLucy was better, it wasn't Lucy Beale). I don't mind recasts as long as they happen before the character is properly developed. Lucy and Lauren both had big storylines, so the new heads weren't believable, to me. I also much preferred the previous actors in the role of Bobby and Peter, but I'm more accepting of them because they were background characters before their new heads.
They can change the head of Hope every week because she isn't a character yet - she's more like a prop - but the likes of Amy and Simon should be kept hold of for as long as possible, because they've brought personality to their roles and, in my eyes, they are Amy and Simon. Anyone new replacing them now would be hard to accept.