colinwalker, this is quite involved and not as easy as key change on a matrix.
You need to edit the eeprom hex. There are different ways, but its not straight forward. The easiest way is using the software i have attached. It's simple software, but you may get loads more questions... sometimes its just easier to use new released pre-edited file, so i'll upload that or mail it to you if u want? I have new one from 2day, and its 2-in-1 for conax and nagra, so may suit u better anyway?