[JimmXinu]

I didn't test it very hard, but it seems to work for me.

Not something I expect to use, personally--I'm more likely to use a keyboard shortcut.

FFF allows for similar change of button type, but doesn't require restart. I have this function:

    def set_popup_mode(self):
        if prefs['button_instantpopup']:
            self.popup_type = QToolButton.InstantPopup
        else:
            self.popup_type = QToolButton.MenuButtonPopup
        for bar in self.gui.bars_manager.bars:
            w = bar.widgetForAction(self.qaction)
            if w is not None:
                w.setPopupMode(self.popup_type)
                w.update()
        return

...and call it from initialization_complete(), library_changed() and ConfigWidget save_settings() after saving settings.