Calculating new differential pressure

The orifice meter would have been sized for a certain flow rate way back when they were installed. During that process, a meter dP range would have been picked depending on plant/company practice and to keep the beta ratio of the orifice plate within certain limits.

The increase in flow rate through an orifice meter as the range increases is proportional to the square root of the dP. To double the flow rate through a meter, you need to take 4 times the pressure drop.

What you need to do is look at the existing meter data sheet and see what the sizing flow rate was and its dP AND the maximum range of the meter at 100% of dP. Then, compare this to the new flow rate you expect to put through your system with the new pump impellers and see if you need change the range and if so, by how much.

Let's say the meter's range is 100" H2O and it was sized for 500 gpm. The data sheet should say either the dP it will have at 500 gpm OR the flow rate through the meter at 100" dP OR both.

Orifice meters are 'typically' sized for normal flow rate to be at about 70% of full scale for flow rate. 70% of full scale is about 49% of full range dP or 49" of water. At full scale reading of 100" of water, the meter will read about 715 gpm in this example.

If you double the meter range (100" of H2O to 200" of H2O), the maximum meter reading is the square root of 2 or 1.41 times as much flow as for 100" H2O or about 1000 gpm (500 gpm / 0.7 * sq root 2).

You likely don't want to operate the existing meters over 90% of full range just to leave yourself some leaway IMO. A meter sizing program will give you slightly different numbers than these as meter coefficients will also change but for liquids, the above approach can be used to pick a range.