WAIT WAIT WAIT!!
You can DO damage with a large water change in certain scenarios:
1- A tank which has had a very low mainteinance for a long time, ie, no periuodical water changes. This can lead to tremendous contaminated water being replaced by very clean water and causing problem to fish.
2- Using water with drastic different parameters (ph, kh, temp), which may shock the fishies.
However, it your tank has been properly maintained, and the water parameters of the new water is similar to old tank water, a large water change should not do any damage. Remeber, periocal water changes is the way to go.