How to change default keyboard on Android N?
It appears that some keyboards offer something under their keyboards (e.g. a special key) to switch keyboards.  However others, such as the Google Keyboard do not.

However, what I have discovered is that whenever the on-screen keyboard is up, no matter which keyboard it is, there is a new virtual button at the bottom right of the screen with the icon of a keyboard.  When I tap this icon a dialog box pops up to change the keyboard.