Just so that we are clear on what you need to do, are you saying that you shot action on a black background, and want to change that background to white? If so, then you will need to Key your action, and place it over a white Color Matte.
If you have your MOV file, that is smaller than the Project's Frame Size, then all you need to do is put that above (higher Video Track) over a white Color Matte.
The Color Matte is available via the New Icon in the Project Panel.
Europe, Middle East and Africa