Lol, who came up with this rule? Pretty silly, if the string is not dead there is no need to change it, much less all of them, its all in your head. Brand new strings sound like a clanky jangly mess to me, dont even get me started on how terrible new strings sound on an acoustic. I shutter whenever people say they like the sound of new strings. Just not my cup of tea. I only change strings when I absolutely have to.