We all do large watwer changes with and without CO2 without issues and have for decades, the pH is not the issue, the KH is.

pH and CO2 can move around, the KH had better not.

Fish can deal with large changes of CO2 levels, which do affect pH, but cannot deal with large changes in say KH, adding a fish from a KH of 1 to 10 will kill them unless done very slowly, and even then.......

pH change is dealing without adding CO2 when it's written about, it leaves out our special case where pH is articifically depressed using CO2 gas.

You can do 90% without any issue as long as the tap 's GH/KH are stable.

Regards,
Tom Barr