Note that the actual change in pH will depend on the buffering capacity of your tank water and your tap water (and because pH is a logarithmic scale). As an example, if your tap water is pH 7 and your tank is pH 8, a 50% water change will probably not yield pH 7.5. Your tank water is likely more buffered than your tap water, so your tank's pH drop will probably be smaller. That said, start with small water changes, give your tank a few minutes to equilibrate, then measure the pH again.
That way you will have more of an idea of how big a water change you can get away with if you need to do it for emergency reasons (ammonia spike, etc...)