This mod requires a cartridge PCB designed for using a 27256 E/PROM and modifying it to use a 27512 E/PROM that contains the two selectable 32k cartridge images. The two 32k images need to be combined first before programming the 27512 E/PROM.

The command to combine two images:

Windows - copy /b low.rom + high.rom combined.rom
Linux - cat low.rom high.rom > combined.rom

The cartridge internals will need to be trimmed as needed to make room for the switch wiring and pull-up resistor.