I think that would be really difficult to do; both the panel and the flight deck would need to be created, as they are different on the early -100.(autopilot and other)Probably not although most airlines did go from the -3/-3A to the -7/7A for some improved performance. I'm recalling most -100's did have those fairly soon when PW made it available. To quote an engineer/pilot/management (not sure) "We had a celebration when the last -3 model engine was replaced by the so much better -7A models."
However, it WOULD be fun to have the -100 with those -3A's that are struggling to perform well. I recall reading from a controller that out of LHR, 747-100's with the -3A's would still be below 10,000 ft by Manchester.Thats 130+ nm away. Slow climb!
It would also require modifications to the fuselage behind the upper deck to account for the hump where the systems are housed.
