The basic bodyshell is the same and possibly the doors I think. However, externally, the tailgate, bonnet, wheels (assuming alloys are fitted) and front bumper are certainly different between the Sharan & Galaxy (the bonnet will be different to accomodate the different shape front grille and headlights and the tailgate different because of the different shape inner lamp clusters etc fitted. The headlights are obviously different, so because of that the front panel may also be different. And I think the mirrors are also different, as well as the external door handles.
Internally, the (upper) dash & storage compartment layout differ, as well as the dial cluster, then you've got the interior trim, although the basic design and layout is similar the trim colours, patterns and materials were unique to each marque, you won't find a Galaxy with the same interior as a Sharan and vice versa.
In short, although the basic mechanicals and body are the same between the two vehicles it seems like a lot of trouble to go to to convert one to the other, to me anyway. It depends on how far you want to take it I guess but if you want a Sharan you'd be better off just buying one that started out life as a Sharan in the first place!
