I have a new gearbox coming for my mount

I am going to attempt to adapt a high precision gearbox to my Losmandy G-11 mount. It is an Ovoid gearbox very similar to the original, except that it has precision gears and ball bearings instead of the plastic spur and no bearing on the shaft. It too is a 25-1 reduction.

The new gear box is coming with a new pinion for the motor. But I have been thinking about this and it seems to me that the programming for the drive would be 25-1 reduction through the gearbox, but with the 18 teeth of the original pinion gear. So it would take 18 teeth to make a complete rotation.

Would this not mean that if the new pinion was not the same diameter and tooth count that the ration would change?
If so, then would this not make it necessary to use the original pinion or else reset the drive programming?

I have been doing some research and it seems that most all the spur gears are a 20° tooth angle. So it would then follow that the pinion should be able to mesh with the new gearbox spur gear.

So, I figure I should see if I can engage the original pinion before attempting to replace it.

Does this seem reasonable to assume? Am I misunderstanding anything here?