Change Window Resolution while Fullscreen

so far i can only see that you can change the resolution of the screen mode you are currently running in. but what if you are running in fullscreen mode, but want to change the resolution of the windowed mode, in an option menu, so that later, when the user hits alt+enter, the correct new resolution will be shown. to be clear, im not asking how to change between fullscreen and windowed mode, and im also not asking how to change the resolution of the current mode. i just want to know how to change the resolution of the mode that isnt currently active.