HobbyBoards did make the change for me. All they did was remove the resistors and bridge the gaps with a piece of wire. The board won't read the pH probe directly. You will need a pH transmitter with 4-20 ma output. This will convert the high impedance signal from the pH probe into a voltage range that can be read by a A/D converter like the DS2450. The DS2450 will read the 4-20ma loop voltage. I found
this with a quick Google search on 4-20ma loop circuit. The trick is to use the right size resistor in the loop based on the voltage supplied to the circuit that will yield a voltage between 0 and 2.5 for the DS2450 to read. The DS2450 can read 0 - 5 volts, but at a much less resolution. The swing of the pH transmitter (at least my cheap one) is less than 2.5 volts, so it's better to use the higher resolution mode. The pH transmitter I got included a span adjustment, so I didn't have to mess with the different resistors to adjust the range to be within 0-2.5 volts. Also, the DS2450 has 4 channels, so you can have multiple pH/ORP probes. I got my pH probe from one of the online reef stores. I think it's a Neptune probe. You'll also need the pH7 and pH10 calibration fluids.
Here is the slope formula I used to get the pH, based on the voltages at ph7 and ph10.
slope = (ph10 - ph7) / 3
pH = 7 + ((voltage - ph7) / slope)
I just found a pH/ORP transmitter on ebay for $45.50 (I'm adding a ORP probe to my system). Unlike the first one I got, this one has an LCD screen.