Been thinking about this problem:
like Karl says, the key is to start with one side of the skin temporarily fastened to (i would say) the BOTTOM of the spar.
then, with the help of straps, buddies, whatever - bend the skin up so its pointing up vertically (with the skin nice and tight against the noseribs on the bottom), and then with a marker, make marks where the front of the noseribs are touching the skin on the inside.
take the skin off, make a line connecting the dots, and then use a long steel pipe of about 1" radius (guess?) to make the new tight bend leading edge round over this pipe along the marker line.
then you should be able to fold over the skin to the top of the sparcap and make your new holes in the skin to make it tight. tighten the whole thing with straps before you make the finished holes.
T.