Can I change preferred operator?

I have a Nokia 6820 with an ATT Wireless SIM card and ATT service. I have
unlocked the phone so it should accept other SIM cards.

The problem is that where I live the Cingular signal is better than the ATT
Wireless signal, but the phone always takes the ATT Wireless signal
preferentially if it can find one. I can't set it to Cingular manually
either unless that is the only signal in the area.

Now that ATT Wireless is owned by Cingular they shouldn't have any
objections to my using the Cingular signal, but unless I change SIM cards
and phone plans to Cingular plans I can't always use the Cingular signal.

Is there some legal unblock or unlock code I can enter to change the
preferred operator? My Nokia unlock program from www.nokiafree.org gives
about seven codes but I am afraid to use them because I don't know if they
will disable the phone permanently.

Thanks.

Ken