That has been annoying me more and more lately....especially when extremely inexperienced people spout off info so matter-of-factually, like they have experienced whatever they were talking about for years. I don't want to be mean because I have done this myself, but it just annoys me now...What happens in theory and in practice are two different things....
Because they don't think about what they say, just repeat "common" knowledge. Some keepers think a large change alters parameters too radically and the fishies don't like it...or they might die or some silly thing. I say it's a lot more complicated than that. See below.
You can absolutely change less (or more) it comes down to experience; where you are on the conscientiousness/ compulsiveness gradient, the health of the fish, level of stock, cost of the fish, level of paranoia and other complicated subjective and objective personal criteria.
I assume someone asking wants a straighter answer, soooo...50%/wk pwc. Can't hurt.
Change as much as you like. My refill water comes temperature-matched from my tap. I just dump it out of the 2g jar into the tank. (A 2g is easier to handle than a 5g bucket.) Sometimes he'll come over and play in the refill splash.