That's not what I suggested - they're too expensive to discard if they're fine. If you were paying me to do a tube swap, I'd suggest doing the lot, but if you've determined which one is trouble, and are perfectly happy swapping it - just do that one and save some money. Only you have control over the sound. It's like the tyres on your car - they all wear a little differently. Do you replace ones that are not approaching the legal limit? I don't know about you, but if I see one is worn, or it fails the annual test, I buy one tyre - I do the other one when it needs it. If you have 4, and one is worn out, replacing that one restores the sound. If you hear other issues, then if they are the same type. NOT similar - tubes fall into a number of categories (Dave is the expert here) but some are acceptable alternatives, and sometime just a brand thing, but others have different performance. I remember the old tales that ECC84 could be replaced by a 6N14P - because it had better performance. Something to do with linear vs not linear gain curves, and they were a plug in alternative some swore by, but an ECC84 had different pin connections to an ECC83 - so swapping to different tubes can be risky if you don't understand them!