Not_a_boffin...
Fitting the APG-65 to GR7/9s would require fabrication of
at least a new front fuselage... every one of the USMC conversions got a
complete new fuselage!
Not very feasible... which is why there was no interest on the part of India.
Now the USMC has about 1/3 of its AV-8B fleet without the APG-65... so they could fairly well replace those with GR7/9s.
However, apparently what they are planning is to transition some F/A-18A/C squadrons to the GR7/9s.
The remaining airframes would be disassembled for components, etc.
Here is what was done in the upgrade:
