A diode placed in the new battery circuit would not keep the new battery from discharging into the old battery. I find the best way to replace the battery in any electronics is to solder an empty battery holder to the circuit board and leaving enough wire on the holder to pull the holder out the chassis. Once this is done put the new battery in the holder and snip the + lead on the old battery as fast as you can. When the new battery needs replacing just pull the holder out and while leaving the power on you can replace the battery without risking power loss to the circuit. The batteries in both the 9xx and the videocipher module appear to last 7 to 10 years depending on temperature and amount of time power has been removed from the unit. Of course if the battery is already dead it's too late to put a new one in as the unit has probably lost it's ID information.
Hope this information will help
