The common sense part applies to the situation where you try to check in or board with a mismatched name. When you call in to CS - irrelevant of the change - they will charge 20 euro. That is the CS fee, and not specifically related to that single character change. That same fee is charged (on top of other fees) when you call in to do any rebooking, upgrading, refund request - basically anything that requires the CS to touch a booking.