Converts
In the NeoGeo community, "converts" commonly refers to modified cartridges, having their ROMs replaced by those of another game or by programmable memory.
This can be done for one's own use, for selling clearly as "converts" (doubtful legality), or for plain fraud.
The usual method is to replace the label on the cartridge's shell, remove the ROMs from the PCB, eventually modify the jumper configuration, and solder back either original ROMs, or freshly programmed EPROMs or flash containing the new game's data.