Not necessary to do a water change, but in effect you are doing one. If for instance the drip acclimation only uses 1 gallon of water, just add one gallon of newly mixed, temp balanced water back to the display.
Leaving the lights off for the rest of the day and the night will be a good thing to help the new fish settle in and the old fish kept in the dark about anything so to speak.
The whole idea is to reduce stress on the new fish so a big water change does not help with stress reduction.
JMO FWIW