Of course if you haven't been doing regular (weekly), large water changes, this will be a shock to the fish's system. If done weekly, you can get away with large water changes& they are quite benefitial. At that point, there is no such thing as changing too much water. Discus breeders do 100% daily. I can't tell you the last time I've had a sick/dead fish (other than of old age). Been keeping fish for 30 years & never done less than 50% weekly WC.
If you haven't done a water change in a while (shame on you), start with 25% WC every other day for a week. Then commit yourself to weekly water changes, for the best health & longest-lived fish.
If you haven't done a water change in a while (shame on you), start with 25% WC every other day for a week. Then commit yourself to weekly water changes, for the best health & longest-lived fish.

