The one visible change apart from the obvious attachment equipment is the supplementary fins attached to the ends of the tailplane on either side. Why were these necessary ?
I've also wondered if the orbiter generates enough lift to offset its own weight. I believe the radome on top of the Boeing AWACS does so.
Also believe the 747 cannot do a transcontinental trip with the orbiter aboard, so Edwards to the Cape has to stop midway for fuel. Wonder if this is a fuel consumption increase or a MTOW restriction.