It is three easy steps:
1. Promote Machine B to domain controller in the domain
2. Transfer all role to Machine B
3. Demote Machine A
See http://technet.microsoft.com/en-us/library/cc526434.aspx for some extra information.
Now machine B is your only domain controller. Don't forget to transfer any other roles you have on there.
As an aside, please stand up two DC's. If your one DC dies, your entire domain is unreachable.
From the above link:
- To start the Active Directory Installation Wizard, click Start, click Run, type DCPROMO, and then click OK.
- On the Welcome page, click Next.
- On the Operating System Compatibility page, click Next.
- On the Domain Controller Type page, select Additional domain controller for an existing domain, and then click Next.
- On the Network Credentials page, enter the administrator username and password, type fabrikam for the domain name, and then click Next.
- On the Additional Domain Controller page, enter the full DNS name of the domain (fabrikam.com), and then click Next.
- Click Next to accept the default database and log locations.
If your domain controllers have more than one hard disk drive, the recommended configuration is to keep the database and the log file on different hard disk drives.
- Click Next to accept the shared system volume location.
On the Directory Services Restore Mode Administrator Password page, enter a strong password, and then click Next.
- Click Next to accept the settings shown in the summary. This starts the creation of the Windows Server 2003 domain controller.
- When complete, click Finish, and then click Restart Now to restart the server.
After the server restarts, log on using an account that is a member of the Domain Administrators group.