Changing GSM band (850 --> 900)

I have two phones running on GSM bands 850/1800/1900: a W380a and a T303a. I'm trying to understand if it's truly possible to convert these so they run on 900/1800/1900 instead.

Have any of you actually done this with a phone, you personally?

On this thread people say you just flash the appropriate regional firmware (i.e. W380a > W380i), but I asked DaVinciTeam about it today and they said "Frequency is tied to the capabilities of the phone's hardware, not its firmware, so flashing cannot change this."

So there is confusion. I hope someone with experience can answer for sure. Thanks!

i have same problem

You can't. Frequency is generated by an oscillator and it is hardware.