How to extend customer master for sales area without affecting other values ?

Hi SD experts,

I had to extend customer master from one sales area (say 0090-DC-00)  to another  (0090-CA-00)  and change sales office / sales group in the 2nd sales area

However, I am now noticing that when I change the sales office / sales group values in 2nd sales area - they get updated for the first sales areas also

I want to keep different values (sales area/group)  in two different sales areas - how to ensure that?

Thanks,