Firstly some mis-information. Black 40/42 was not the start of GE engines.
Secondly, there is a reason the US moved to GE engines and most operators today prefer GE.
Newer blocks can switch between engine types, not sure this is true for the earliest blocks.
American engines use similar tech and maintenence systems which are different from Russian or Chinese ones. It is not the biggest deal for Pak engine maintenance crew to switch from one to another.
GE is superior tech to PW but PW is slightly lighter.
@Khafee has already said PAF is likely to prefer the PW engines. He is simply noting his disagreement with that decision (when it happens)