Airplane Conversions with Multiple Engines Available

[swiftus27]

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.