So long as the new water is within 15 degrees F of the tank water, nothing bad will happen. Even if you do a 50% change, the net result is water that is only 7.5 degrees from where it was -- a small amount really. Of warmer or cooler, it is probably best to err on side of cooler. Of the temperature changes that occur in nature, temperature drops are more common than jumps. In fact, you may even see some new behavior from your fish, as for some species, a rush of cool water represents a spring time melt, and triggers spawning events. I am never seeking spawning behavior, but at least a few times each year, my cory catfish will spawn after a water change with cooler water.