Here is what I was looking at starting with when I had vans from both years. 2002 Inner door on left, 2003 Inner door on right (or center). 2002 Outer sheet metal is the third picture where you can see the additional holes for a 2003 mirror are missing from the OUTSIDE.
The 2002 mirrors have very short studs with nuts and washer that are tensioning against the outer door sheet metal. The 2003 mirrors have much longer studs and the door sheet metal has a recessed piece tacked to the inner sheet metal that the nuts/ washers tension against.
The 2003 mirror going into a 2002 door is going to present studs that are set into bosses that are designed reach the 2003 inner sheet metal piece. The holes for the additional studs can be opened up in the outer sheet metal so that is relatively easy. When it comes to the studs, the 2002 door leaves those 2003 stud bosses hanging in air with nothing to tension the nuts against. This is the challenge in swapping the '03 mirrors to the '02 doors.
I never went any farther as I actually came to appreciate the 2002 wider stock mirrors vs the the 2003 and up Tow mirror (unless I was actually towing a wide trailer). The 2003 stock non tow mirrors are actually taller but not as wide as the 2002.
It looks like Lilnuts2 removed the studs from the bosses and used button head screws and washers tensioned against the inner sheet metal. This would allow the plastic trim piece to fit back into place due to the low profile of the button heads.
I am not sure how CarringB attached secured the nuts.
One thing that has been determined is the OEM Mirrors are much preferred over the aftermarket units.