this is the same as the advanced class topic. it doesnt need to be in the game. it says when you are picking it. you can not change it. and nor should you be able to. people just need to make better choices when playing there characters.
Actually when you first picked your legacy it didn't tell you it was for all your characters, many people believed it was a surname for the character you was playing not for every character on the server.
I think a one time only rename should be viable option, maybe something you could spend some legacy levels on?