Why would it alter the ph? Presumably you're putting in the same water, from the same source, every time you do a water change. The ph of my water supply is very consistent, right around 7. Is that not the case for you?
But every serious fishkeeper does big weekly (if not more frequent) water changes. It's not optional regardless of ph.
If he hasnt changed water for som time the ph of his tankwater drops due to being polluted by fishwaste. And when doing a large wc he puts in water with higher ph and it fluctuates wich can directly kill the fish. So if it where me in that situation id rather do 20-30% wc every day for a couple of days instead of one large wc lets say 80%. By doing small frequent changes hes not altering the ph to much at a time.