Amusingly, DL does accept my answer "Han må bytte bokstaven", but evidently it's a pretty different meaning from what they had in mind to begin with!
What is the difference between endre and forandre?
I think it's like the difference between "to alter" and "to change".
Is 'bytte' wrong here because it means 'exchange' or 'replace' rather than 'alter'?
Yeah, we didn't think of the other meaning of change. We've added it now.