We help IT Professionals succeed at work.

Merging two Companies with 2003 Active Directory to 2012 Active Directory

I have the following situation:

1. Company A (a .local domain) with 2003 Active Directory and hosted Exchange 2010 Server
2. Company B (a .local domain) with 2003 Active Directory and local Exchange 2003 Server

Company A is merging with Company B - and we would like to have an entirely new .local domain with a Windows 2012 Active Directory Domain Controller, and Office365 Exchange server.

There are SO many variations of this problem here - that I'd just like to get a start going in the right direction.  I want to continue to allow users to have access to the network shares that they currently have access to as well (Company A has a NAS server, Company B has a Windows Storage Server).

Can someone help me get started?

Watch Question

Mai Ali (MVP)Senior Infrastructure Consultant

You will need to do forest trust between to company to allow users to have access to the network shares after merging.
Senior Solution Architect
Most Valuable Expert 2015
Top Expert 2015
This is quite the question. High level steps are below...
- Create a new Forest root domainC.local
- Create 2way trusts from A to B and B to C and A to C
- Use the ADMT Tool to migration AD objects from domain A and B to C (ensure you are using sidHistory when migrating)

For Exchange
I would recommend migrating both 2003 and 2010 mailboxes directly to O365. Use the Microsoft Exchange Deployment Assistant to accomplish this.

Microsoft Exchange Deployment Assistant

Helpful Tutorial

As stated these are highlevel steps

Once all of your AD Objects and Exchange mailboxes/DL's ect have been migrated you can decommission domain A and B and remove the trusts as well.



Great (and simple) solution Will.  Any issues using ADMT tool from 2003 to 2012?  I thought I read that this didn't work to go directly to 2012?

Will SzymkowskiSenior Solution Architect
Most Valuable Expert 2015
Top Expert 2015
ADMT 3.2 is the most current version and as long as you have a 2003 Forest Functional Level you can go from 2003 to 2012 with out issues. See below link for details.

Distinguished Expert 2019

Just for curiosity:
Why you want to build 3rd .local domain
U can put any one domain out of TWO (say a.local), migrate it to O365 and then merge another domain (b.local) to that domain

Also I don't think another domain mailboxes will get migrated directly to your O365 subscription
Because one O365 account would not be synced with TWO ADs same time

You should 1st migrate another domain to yours with mailboxes and then finally move all your domain mailboxes to O365 (recommended)
With brand new 3rd forest with exchange, you should move mailboxes from other two forests and finally move 3rd forest to O365 (less recommended)

Check below threads

Also note that ADMT doesn't migrate Exchange attributes, hence you need to use either PowerShell scripts to provision cross forest accounts in advance or use FIM to provision them


Mahesh - great comments and gives me pause to think about this more.  I need to create a third .local domain (I think) because it will be a different name.  I can't use CompanyA.local or CompanyB.local because I need to rename the company (the merger created a newly branded company).  I'm still concerned about the hows (and timing) of moving the mailboxes and still maintaining email continuity, etc.

Here are my thoughts:

1.  Create new Windows Active Directory 2012 with new company name (CompanyC.local).
2.  Create two way trusts with existing CompanyA and CompanyB 2003 AD Domains.
3.  Migrate CompanyB users (with on-premises Exchange 2003) to CompanyC domain using ADMT.
4.  Create Office365 Exchange hosting for CompanyC, and migrate Exchange 2003 users from CompanyB (previously moved to CompanyC) into Office365 Exchange using AD Synchronization and a cutover migration (?).
5.  Migrate CompanyA users (with hosted Exchange) to CompanyC domain using ADMT.
6.  Migrate CompanyA users hosted Exchange to Office365.


Distinguished Expert 2019

If you must built 3rd domain, you are on right track

Some important things:
If this domains have big user base to migrate, consider deploying FIM to provision user accounts, groups and cross forest contacts (GAL Sync) in advance
This will automate migration process and GAL synchronization


You need to use Exchange preparemoverequest.ps1 script which will convert contacts to mail enabled users and then you can move mailboxes

Lastly you can use ADMT to migrate those accounts in merge mode so that SID History will get migrated as well

Note that ADMT cannot migrate Exchange attributes, hence you need to create those accounts either with FIM or preparemoverequest.ps1 script 1st and then merge SID History with ADMT

maintaining co-existence (mail flow) across multiple Exchange organizations including O365 is the major part where you need to work \ design