How to demote backup Domain Controller

I have a Windows 2003 server as DC and two App Servers running W2003 Server. One of them is acting as a backup domain controller. I want to disjoin the backup DC from the old Domain and join the new domain.
However when I open My Computer Properties to disjoin the server, I get the message seen in attached.
I understand that I need to demote this server so that it does not act like a backup domain controller.
However I wanted to following steps given by experts so that I don't screw up things on this app server.
There are two application software that users depend on from this server, so I need to be careful.
Client programs on workstations access the data on this server by NETBIOS Computer name or IP address, so I think it would be alright to move this computer from old domain to the new domain as long as there is no change in IP address or computer name.

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The proper was is to use dcpromo and don't select the last dc in the domain.
Join the new domain.
sgleeAuthor Commented:

dcpromo failed with the message:

The operation failed because:
A domain controller could not be contacted for the domain XXXX that contained an account for this computer. Make the computer a member of a workgroup then rejoin the domain before retrying the promotion.
"The specified domain either does not exist or could not be contacted."

Strange thing is that I can ping the DC either by computer name or IP address. Another thing is that this computer has been acting like a sick puppy - slow at times for no apparent reasons.

What should I try next?
sgleeAuthor Commented:
here are screenshots leading up to the error.
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Create a reliable backup. Make sure you always have dependable copies of your data so you can restore your entire system or individual files.

Check the DNS settings in TCP/IP.  The Primary DNS server should be your server with DNS for Active directory, probably your Primary Domain Controller.

Also, you should check that all of the FSMO roles are already held by the Primary domain controller.  If any are held by the backup DC, then you should move them to the Primary DC before you demote.

this article shows 1 way to check for the FSMO roles

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sgleeAuthor Commented:
As you suspected / suggested:
When I checked DNS server on TCP/IP setting on this backup DC, it showed the IP address of new DC with new domain name. What happened was that I created a new SBS2011 Domain and joined all workstations into the new domain. I just kept a few 2003 Servers joined to old domain name.
I did change DNS IP address so that it points to old DC and I was able to run DCPROMO.
It has been running about 30 minutes and no end in sight. As seen in attached, it has been securing different folders in C Drive.
I will keep you up-to-date tomorrow AM.
sgleeAuthor Commented:

 That was it. correctiing DNS IP address from new DC to an old DC did the trick.
 Thank you very much.
sgleeAuthor Commented:
chakko was right on it.
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Windows Server 2003

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