Amras Felagund said:
Perhaps the "Luke and Leia" theme could play when Luke contacts Leia from under Cloud City, in place of the Force Theme. Of course, the original music could "bleed" through under the new music, so... I'm not altogether sure about this. It could work, but still...

That scene as it is with the force theme going is so powerful, I don't think it's a good idea to change it.

(but I guess I'd have to take a look at that with different music first to be 100% sure of my opinion)