Eric is correct,
All of the different ratio gear boxes have the same spline (28T, 0.5Module Side Fit DIN Spline) coupler. The input housing, output housing, input coupler & bearings, and output bearings are identical for all gear boxes. The ring gear is also the exact same no matter what gear ratio you are using. The design is such that you can swap any ratio at any time and it just works. In addition, the direct coupler feature was designed in because we have always had issues on my team with trying to prototype using the smaller motors.
Allan’s instructions are good and we also put the conversion instructions in our full user guide which should be out tomorrow. You just remove the ring gear, sun gear, carrier with planets and assemble the input housing directly to the output housing.
The shafts are also interchangeable so if you buy a 3:1 or 4:1 and want a 1/2" hex output shaft, just buy the hex output shaft separately and install it into the output housing.
Paul