Changing 6230b firmware - 850 Mhz to 900 Mhz

[wjlost]

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