I want to find a way to programmatically fire the selector that causes the iPhone keyboard to switch from letters to numbers. I am aware that I can switch the keyboard type, but I want to know if there is a way to do this without switching the keyboard type.
You cannot switch keyplanes (alphabetic keyplane to numeric keyplane to symbol keyplane) programatically, at least not in any way that is publicly supported. As you mentioned, you can change the keyboard type or appearance of the text input traits of the first responder, but this is different than switching between the different keyplanes, which only the user should be able to do.