all you do is switch the SIM card from one phone to the other. Cingular uses 850 MHz band, so make sure the phone has this band, and I strongly believe the Motorola V620 is a quad band phone so you should be OK.
If you go to a Cingular store, they will try to convince you to buy a phone from them, so don't go there.
I currently have an LG G4015 and im thinking about getting a Motorola V620. I want to stay on my same service (Att/Cingular) but just want to switch cell phones. From the sites Ive been to it seems like all you have to do is take the SIM card out of one and put it into another. Is that true? or am I going to have to go into a Cingular store and have them switch things for me?