Sony Ericsson P900

I am fairly new to the GSM world, and have a few questions. If I get an "unlocked" version of this phone, and presently have Cingular GSM service, what would I need to do, to scrap my current phone and get this one activated? Would I have to take it to Cingular to get it programmed or just plug in the SIM card from my existing phone? Also I've noticed this one operates on 900/1800/1900 MHz as opposed to most Cingular phones (850/1800/1900). Thanks for your insight,

Mohammad Raqib