I think the SRs and Ds have the normal range of a 747, but have considerably beefed-up undercarriage and critical structure sections to counter the stresses of higher frequency operations. In fact, airframe life is dictated by two things: total hours and cycles (a cycle is a take off and a landing). Off the top of my head, I think a 747 is designed for 40,000 cycles and 100,000 hours. In short range ops the 40,000 cycles would be reached long before 100,000 hours, making retirement a tad premature. Therefore the -400Ds are designed to be retrofitted with winglets and put into long-haul service when half the cycle limit has been reached, so that considerably more hours can be squeezed out of the airframe (at which point JAL and ANA will order new 747-400Ds or A3XXDs).
Perhaps the same decision has been reached with the -100SRs, -200SRs and -300SRs, to put them into long-haul service in the relative twilight of their careers, as the airframes must be rapidly approaching the structural limit on cycles but have many hours left on the clock.
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz