Water changes work if you're on city water, otherwise there's no power to the well pump.
If it looks like bad storms are headed in and a scheduled water change is in the near future, doing it sooner than later is a good idea.
Once the power is out, don't feed the fish IMO. It'll only contribute to ammonia buildup.
Also try to avoid startling them - the more active they are, the quicker O2 is consumed.
O2 moves into water when there is a difference in concentration. If there's less O2 in the tank because of the fish using it up, the O2 in the ambient air will move into the water. Sometimes there can be a protein film that builds up on the water's surface that blocks this process. Thus the talk of how water disturbance is important in keeping aquarium fish. If the power has been out for an extended period, occasionally swirling the surface of the water can help breakup the film and allow O2 to pass.