I saw how in the Bachmann order parts section of Devious Diesel. There's the face plates of both the regular and grumpy Diesel models. Is it possible to interchange between these faces?
Yes, it is possible. All you need to do is carefully pop the face off your Diesel model using a hobby knife, and then switch to the expression you want the model to have. When putting the face back on, make sure the eyes line up with the mechanism, as this will allow them to move while the model is running.