Note that in Windows 2000 and newer, there isn't really such a thing as a
backup DC. All DCs are equal peers, with the exception of the 5 FSMO roles,
and the global catalog(s).
Before you demote a DC, you should make sure it doesn't hold any of the FSMO
roles and isn't the only global catalog on the domain. By default the first
DC on the domain holds all of these roles, unless maybe you or another admin
moved them to another DC. It's probably not a bad idea to consider having
more than one DC have a global catalog. If a DC with one or more of the
FSMO roles is lost, there are ways to recover those roles, but there can
still be problems. Google "FSMO site:microsoft.com" if you need to find the
MS KB article on how to check which DCs have the FSMO roles.
Can a bkup DC be demoted to member server?