Actually the -200 SUD came out after the 747-300 was designed. Since there was little difference between the -300 and the -200 (aside from the longer upper deck) it was a relatively cheap way to get the benefits of the -300 without buying a new airplane.
IIRC, Boeing did the -200 SUD retrofit on 20 airplanes.
BTW, the longer upper deck didn't just add some seats - it gave the airplane better area ruling leading into the wing which improved the high speed drag characteristics. Improved cruise Mach by about .01
