Changing 6230b firmware - 850 Mhz to 900 Mhz

Hi,
I recently bought a Nokia 6230b, which supports GSM 850/1800/1900. I would like to tweak its firmware to use GSM 900 instead of GSM 850.

Can this be done?
Any tips and help will be appreciated.

Thanks,
Reuven

nope, thats why they sold the 6230a version, because its a hardware difference, not software