I think you can copy the injector data from the GPCM to the ECM, then when you get the new GPCM in, copy it back from the ECM to the GPCM.
I want to say there is already a copy in the ECM, so you might be able to just put in the new GPCM and copy the ECM values to it.
Though all this is based on my recollection of LBZ/LMM, I haven't touched a LML before.