Had a quick look at hex editing... trialed on VBOX image only as getting late....
Seems a quick change (only for the last number in the serial) that works with the check sum is that the number after the serial number needs to be 1 higher.
e.g. if your serial is B3J4N00000 then the version in the file will need to be B3J4N000001
So, if you want your serial to be B3J4N00008 then the version in the file needs to be B3J4N000089
...not tried editing the IMG file yet, but assume the same will work.....