We have a Windows 2008 domain, with a Windows 2008 Backup domain controller that is nearing end of life.
What is the proper way to demote or inform Active Directory that the server is going to be removed from the domain.
As others have said:
BDC's haven't existed since Windows NT 4.
If this DC was the second or subsequent DC in the domain, and if you have not manually moved any of the FSMO roles to it, then running dcpromo is all that is required to demote it. If you have manually moved any of the FSMO roles to it you'll want to move them to another DC before running dcpromo.
If this DC was the first DC installed in the domain, and if you have not manually moved any of the FSMO roles to another DC, and if you have another DC, and if all DC's are operating normally, then the dcpromo process will handle moving the FSMO roles to another DC without any intervention from you.
In most cases, no manual moving of the FSMO roles is neccessary unless you have a need for a particular role to be on another server or you're recovering from some type of DC crash on a DC that holds any of the roles.
You should also bring another DC online before starting the whole process.
Then you need to make sure it is not a role holder or performing any services. Then simply run dcpromo and remove it from the domain.
As others have said, running DCPROMO should be enough to properly remove that DC from the domain.
If the DC is a DNS server, you should take care that