What causes for the ac air to change from the front vents to the dash defroster vent when you accelerate on a 1993 Ford van?

Low vacuum. When you push down on the accelerator, vacuum decreases. Your Blower doors use vacuum to change positions from floor to vent to defrost. Have a vacuum test done on the vehicle. You may have a vacuum leak, or this may be a sign the engine is getting weak.(Low compression, Leaking valves, etc.)