Add the new doctor in PW as an employee, and if appropriate select the default checkbox.
Go to the previous doctor's employee record (List > Employee), and open appropriate record.
Uncheck "Can be a primary Dr," and click OK to close. When you do, PW will ask who should now be the primary Dr for the corresponding patients. Select the correct Dr, and click ok. It will change all primary Drs from one to another.
If this change is being made because of a Change of Ownership, please contact support so that we can get you on the right track with getting everything switched over.