So why continue with it as selling pre-burned "ROM" chips? Make a board that carries yor original 3.1 ROM and a flash chip, like a kick switcher. When writing to the ROM addresses, obviously you're only able to write tot eh flash, so do that, even if you booted from the original 3.1 ROM.
Make a software utility that reads in your real ROM, asks for your 3.9 CD or looks on your hard drive OS install (which may be better since boing bags and other udpates are not on CD), and read all that to assemble the 3.9 "kickstart" image which is then burned to flash. Next boot, you have a 3.9 to choose from in your now programmed flash side of the kickswitch, and have a way to mark the flash side as the default boot from now on.
Do you need to ask for permission and pay license fees this way? You already have what is needed, you just need to assemble it and put it inside the flash yourself, and the utility makes that very easy to do, pretty much automatic.
