In preparation for Exchange 2016 I had to remove a 2003 DC from my environment because Exchange 2016 requires a forest functional level of 2008.
There are guides available on how to decommission a Domain Controller so I won't go into that. I just want to touch on a small little error I received and how to fix it.
During my dcpromo portion of the decom I ran into a NETLOGON error. This is due to the machine you're trying to decom is still pointing to itself for DNS. To fix this simply go into your NIC settings and change the DNS to point to the new DC.
Sometimes things are easy I guess!