Cod ww.... I believe has a prompt for 3.15 or 3.40. The problem here is you cannot go backwards with your FW updates. If your on 3.41 (which I imagine you are if your Jailbroken) the only place to go is up with your firmware.
So your mentality is to switch FW to backup this game with the games FW prompt is good thinking, EXCEPT this game requires lower FW and your JB device will not be recognized on any other firmware but 3.41 to allow recognition.
On the comparability list it said it had been copied, there may be versions of the game that don't have the mandatory update.
The crazy thing is you will have to just let that game beat you this time until we figure a way to JB on other firmwares. Hope this helps ...GOOD LUCK!