The 6820 will work with any company (even Cingular) when you unlock it.
A google search will come up with many sites that give you instructions
and unlock codes. Be careful with this, if you enter the code wrong to
many times the phone becomes useless. If it fails a couple if times,
you should take it to a pro to unlock it. Around here they charged me
about $30 to unlock the phone.
The phone will work as long as the SIM card is active. Have Cingular
give you a new SIM card to put on the phone so it works with your new
Cingular account. If you are renewing your contract, you might as well
get a new phone and just keep the SIM card and sell the new phone. Hope