edit a firmware from the Nec UPD78P4038GC microcontroller, so that it works with UPD78P4026GC

I'm looking for someone who can edit firmware from a Nec UPD78P4038GC microcontroller so that it works with Nec UPD78P4026GC. These two microcontrollers are almost identical, they are pin compatible, but their firmwares are not interchangable out of the box. There are no source codes for this firmware, so it is not possible to compile this firmware for another microcontroller.