Just a word of caution. Some pump manufactures will simply change out the impeller of a 3/4 HP pump to 1/2 HP impeller but use the same 3/4 HP motor. The result is an under loaded motor which fairly inefficient. You can this is the case when the maximum current draw is the same for a 1/2 HP and 3/4 HP pump.

It sounds as though you have about 50 feet of head with your current pump. However, the head loss will change with a new pump depending on if the new pump has a higher or lower brakiing HP (BHP). A fairly good approximation for the head loss change is:

Head Loss New Pump = Head Loss Old Pump * (BHP(New)/BHP(Old))^2/3