How to change icon in command after it is created?

Hi,
In my toolbar, I want to have a login/logout button. The button goes from logout to login state and vice versa. Both the state has a different icon. I tried changing the icon but it did not work.
Also, what does MF_DISABLED do? I can click the button in the disabled state.

I am afraid you cannot change the icons.

Are you on Windows or Mac?