Maybe the best way would be to extract the software from a new version 7900 and then use that for the first run radio? I'm no guru by any means.First step towards doing that is to gather data about the chips in the radio and figure out how to interact with them. Since any new firmware would probably be pushed over a USB programming cable it could be possible to extract it the same way, but you'd need to know what the hardware is to know how to do it if it can be done.
Then you have the problem of can what was dumped be understood and can it be modified? There are various ways of preventing reading and/or modification including encrypting stored data or at least having a cryptographically signed checksum. I don't know if the vendors go to this much trouble but it is something to be aware of.
I think that's why Mark19960 chose to build his own board for the Uniden 980. Couldn't get at the firmware directly so he substituted his own along with the hardware. The vendors don't want us playing around at that level. The smart ones are releasing updates so we don't feel like we have to.
Mark19960 just released a video doing the software swap between two of the newest version 955's.