Just an example:
If I were to convert a Boeing 720 or a 707-120 to a B, I feel that I should be able to use any engine that is currently available at that time. Silly I should be forced to use the JT3D-1 when the -3 is an available option.
I guess similarly one should do the same with DC 8s when they go to the -70s? I dont know for sure.