A merger means we must rename or merge our AD domain.
Domain rename (rendom.exe) is technically possible but painful: Exchange doesn't support it, certificates break, applications need reconfig. For most mergers, building a new domain and migrating with ADMT is cleaner.
Try this first
- 1Inventory both sides: forest/domain functional level, FSMO roles, applications, integrations (Exchange Hybrid, Entra Connect, third-party identity).
- 2Decide: rename (rendom) or build new and migrate with ADMT/Quest/BitTitan. With Exchange or Entra in play: almost always migrate.
- 3Establish a trust between old and new domain. Migrate per OU using ADMT, keep SID history.
- 4Plan a dual-account period: users temporarily have accounts in both domains, applications get moved to the new domain.
- 5Decommission the old domain only after months of stability and confirmation that everything runs. Remove trusts in the proper order, all objects migrated.
When to bring us in
A merger where AD architecture goes wrong is a drama that lingers for years. For larger environments or multiple DCs per side, hiring an experienced consultant is almost always cheaper than muddling through.
See also
- One DC or two DCs for an SMB office?Two is almost always the right answer; one DC is a single point of failure for logon, DNS and GPOs.
- Should I split FSMO roles across two DCs?For a small domain all on one DC is fine; with two DCs splitting is tidier but not required.
- How do I know my AD replication is healthy?Replication errors creep in silently; they only surface when logins or GPOs misbehave.
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