Larry, how did you come up with "8.48"?You can, but I would not recommend doing it. How big of a water change are we talking about? If you did a 20% water change with water that is 8.8 pH and your tank is at 8.4, the water change will only bring the pH in the tank up to 8.48.
I guess the easiest way to explain it doing a 20% change is....
8.4pH x .8 (80% of remaining water) = 6.72
8.8pH x .2 (20% of new water) = 1.76
100% = 8.48 pH