Replacing backgrounds with those from ESB can only be possible if the scene had locked-off cameras like Luke & Leias Death Star swing and the least amount of masking & keyframing to keep it believable, Luke's Lightsaber training on the ESB Falcon would be pretty hard with all the motion blur etc (i'm not sure if the ANH footage of Luke is locked-off) and thats just one person in that shot with its background replaced.
It would be very time consuming.
J