As RobertoS has said, the O2 Wireless Box remains the property of O2, apart from that if you have a problem with your connection they will expect you to have it connected so they can carry out tests, at which point they would probably know it is not running O2 firmware.....
All that aside, this thread has explicit instructions, I didn't bother following in the end as mine is working fine now with the BE firmware....
http://forums.modem-help.co.uk/viewtopic.php?t=2877