Shot Drop factor:
 On the trap field when you back up to that 27 yard line most folks will start reaching for a load of 7-1/2 super killers, instead of the proper choke tube for that range, choke tubes where meant to be used so you can change range patterning of your shotgun.

 The shot does drop:
 Now lets look at the difference in drop and energy factors of these loads of #7-1/2, 8, 9 and 4 shot. Velocity factor of 1200 feet per second range 60 yards.
 Drop factor:
 (7-1/2) drop - 10.9" from bore line.
 (8) drop - 11.3" from bore line.
 (9) drop - 12.4" from bore line.
 (4) drop - 8.8" from bore line.
 Energy factor:
 (7-1/2) .85 ft.lbs. energy per pellet at 60 yards.
 (8) .68 ft.lbs. energy per pellet at 60 yards.
 (9) .42 ft.lbs. energy per pellet at 60 yards.
 (4) 3.01 ft.lbs. energy per pellet at 60 yards.
 Pellet count 1-1/8 oz. load:
 (7-1/2) 393
 (8) 462
 (9) 658
 (4) 152
 Arrival time to 60 yards in seconds:
 (7-1/2) .2375
 (8) .2423
 (9) .2538
 (4) .2140

 Overview:
 As you can see here when presented with the facts there is NO REASON to swap shot sizes in the clay games when the yardage changes. Screw in chokes where designed specifically for these changes in yardage and besides no two shot sizes will pattern the same in the same choke in the same shotgun.