Switching from Google Japanese Input directly to Google Keyboard?
AFAIK, this is impossible to do in KitKat and previous version. There is no globe (input method switch key) icon, and there is no setting related to it.

However, recent update added this feature, BUT it's only for Android 5.0 Lollipop (as of writing). From "What's New",


  (Lollipop) Introduced input method switch key.


On Lollipop, there will be a globe icon on each type of keyboard:


12 Keys: On Japanese mode, to the left of 'わ'.
QWERTY: On Japanese mode, to the left of the spacebar
Godan keyboard: On Japanese mode, between 'O' and 'W'