How do I promote a Backup Domain Controller to the Primary Domain Controller?

A. When possible you should always promote a BDC to the PDC while the main PDC is still active, in this way the original PDC will be demoted to a BDC and no information will be lost, however sometimes the PDC will not be available (i.e. its crashed) and a BDC needs to be promoted, as in the absence of a PDC, a BDC does not automatically promote itself.

1. Log on to a BDC as an Administrator
2. Start Server Manager (Start - Programs - Administrative Tools - Server Manager)
3. If the PDC is not available then a warning will be displayed "Cannot find Primary DC for <domain>"
4. Click on the BDC you want to promote to the PDC
5. From the Computer menu select "Promote to PDC"
6. Again if the PDC is not available then a warning "Cannot find Primary for <Domain>". Click OK to continue
7. The Netlogon service will be stopped on the BDC, it will be changed to a PDC and then the Netlogon service will be started again.
8. This machine is now the domain PDC

If the PDC was online, it would automatically be demoted to a BDC when the promotion occured.

To bring the original PDC back if it had not been demoted prior to the upgrade just start as normal, it will detect the running PDC and stop its netlogon service. You can then start Server Manager and select "Demote the BDC" and then promote.