make existing rdesktop session grab keyboard/mouse

By default, rdesktop grabs keyboard, so that key combinations such as Alt+Tab or Alt+F4 act inside the rdesktop window. This can be overridden with the -K option rdesktop -K myserver.mydomain.com Is there any way to chage this during an existing session? I always connect using the -K option, but sometimes I would like to switch so that rdesktop grabs my keyboard.