Migrating a Domain Controller (DC) from Windows Server 2016 to Windows Server 2025 requires careful planning and execution to avoid disruptions. Below are the steps to perform this migration:

Preparation

1. Check System Requirements
   • Ensure the new Windows Server 2025 meets the hardware and software requirements for Active Directory Domain Services (AD DS).
2. Backup the Existing DC
   • Use Windows Server Backup or a third-party tool to create a full backup of the existing DC, including system state.
3. Update Windows Server 2016
   • Ensure the current DC is fully updated with the latest patches and service packs.
4. Test Environment
   • Set up a test environment to simulate the migration if possible, ensuring there are no surprises during the actual process.

Migration Steps

1. Install Windows Server 2025

• Install Windows Server 2025 on the new server and configure basic settings (IP, DNS, etc.).
• Join the new server to the existing domain as a member server.

2. Install Active Directory Domain Services (AD DS)

• Open Server Manager on the new server.
• Add the AD DS role by navigating to Manage > Add Roles and Features.
• Complete the wizard and allow the server to restart if prompted.

3. Promote the New Server to a Domain Controller

• Open Server Manager and click on the warning notification for AD DS.
• Select “Promote this server to a domain controller.”
• Choose to add a domain controller to an existing domain.
• Provide domain credentials for promotion.
• Select roles (Global Catalog, DNS Server) and choose to replicate from an existing DC.
• Complete the wizard and allow the server to reboot.

4. Transfer FSMO Roles

• Open a Command Prompt or PowerShell as an administrator on the new DC.
• Use the following commands to transfer Flexible Single Master Operations (FSMO) roles:powershellCopy code Move-ADDirectoryServerOperationMasterRole -Identity "NewDCName" -OperationMasterRole PDCEmulator, RIDMaster, InfrastructureMaster, SchemaMaster, DomainNamingMaster
• Verify FSMO role transfer using:powershellCopy codeGet-ADForest | Select-Object SchemaMaster, DomainNamingMaster Get-ADDomain | Select-Object PDCEmulator, RIDMaster, InfrastructureMaster

5. Decommission the Old Domain Controller

• After confirming the new DC is functioning correctly:
  • Open Server Manager on the old DC.
  • Remove the AD DS role by navigating to Manage > Remove Roles and Features.
  • Follow the wizard to demote the old DC.
  • Ensure it’s removed from the domain.

6. Clean Up Metadata

• Remove references to the old DC in Active Directory Sites and Services:
  • Open Active Directory Sites and Services.
  • Expand the site containing the old DC, right-click the server, and choose Delete.

Post-Migration

1. Test Functionality
   • Test domain authentication, DNS resolution, and group policies on the new DC.
2. Update Clients
   • Update DNS settings on clients if they are pointing to the old DC.
3. Monitor the Environment
   • Monitor for replication errors or login issues using tools like Event Viewer and dcdiag.
4. Document the Changes
   • Record all changes made during the migration for future reference.