Can I improve efficiency by moving air ducts from attic to crawl space?
The first step is always to lower where the heat escapes into the house (for summer cooling season). You most likely have plenty of attic insulation.  But you might check for other air leakage places (around doors, windows, pipes, electrical outlets, etc.)  Less heat entering is less heat to cool.  Shades or solar film over windows help too, as well as trees etc.

If the air ducts are insulated, then there might be some difference in moving the duct work to the crawl space, but most likely not enough to cover the cost to change it.  If they are not insulated, then the cost of installation still might be higher than many years of savings.   

Another possibility for savings might be an attic vent fan, which could lower the attic temperature some.  Just lowering the attic temp by 10 or 20 degrees is worthwhile in many places.  It means a little less heat getting through the insulation and the ducts would absorb a little less heat as well.