switchingwidgets.kv

#:kivy 1.5.1
#:import kivy kivy


<FirstWidget>
    GridLayout:
        size: root.size
        rows: 2

        Label:
            text: 'First Widget'

        Button:
            text: 'Go to Second'
            on_press: app.root.widget_switcher('second')

<SecondWidget>
    GridLayout:
        size: root.size
        rows: 2

        Label:
            text: 'Second Widget'

        Button:
            text: 'Go to First
            on_press: app.root.widget_switcher('first')

switchingwidgets.py

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.floatlayout import FloatLayout


class FirstWidget(Widget):
    pass

class SecondWidget(Widget):
    pass

class SwitchingWidgets(App):
    def build(self):
        parent = FloatLayout()
        self.first = FirstWidget()
        self.second = SecondWidget()

        parent.add_widget(self.first)

        return parent

    def widget_switcher(self, string):
        self.root.clear_widgets()
        if string == 'first':
            self.root.add_widget(self.first)
        elif string == 'second':
            self.root.add_widget(self.second)

if __name__ == '__main__':
    SwitchingWidgets().run(