Yes you can debrand a biggi too, many have. I haven't so I can't comment on how it all works tho. XDA developers site is another good one to get info on doing techy stuff to the phone if you want to gather a bit more info before doing anything to your phone.
Sony Ericsson XPERIA X10 - xda-developers
Have you tried updating your phone as the most recent firmwares are R2BA02* the last digit seems to depend on location and carrier.