Moving existing DC to new domain

My business has just acquired a new office and will be taking over their technology and network configs.  They have a domain controller which I would like to join to our domain.  I've joined brand new DCs to my domain before, but never an existing DC from someone else's domain.  Is the process any different?  Are there any pitfalls I should watch out for?

My domain already has 2008R2 as the primary DC.  The DC being joined from another domain is also 2008R2.

Any insight people can provide would be much appreciated.
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Lee W, MVPConnect With a Mentor Technology and Business Process AdvisorCommented:
You cannot join an existing DC from another domain into your domain.

Your options include:
1. Create a trust between the domains (unless either network contains a Small Business Server - Domains with SBS cannot have trusts).
2. Use the Active Directory Migration Tool to migrate the users and computers from the domain with the foreign DC into your domain.  (I don't like doing this; I prefer creating the users new and migrating the profiles - it creates "baggage" that I don't like having on the accounts - though I've not seen anyone else really back me up on this or any other supporting documentation to say it's "bad").
3. Demote the existing foreign DC (this loses all account information assuming it's the only DC) and then join it to your domain and promote it).
Radhakrishnan RConnect With a Mentor Senior Technical LeadCommented:

Since you are going to add their existing DC into your domain? i believe that they must be having a domain? are you going to leave that domain and keeping only your domain? what about their existing users?

If they need their domain then the good option would be create a trust between these 2 domain.

Or you will need to transfer all the accounts, data onto the your DC and disjoin the DC to workgroup and add this back to your domain.

Domain and forest functional level remain same as it is 2008 R2.

I think you need to plan very well before making any changes on the existing DC.
ccoldsmokeAuthor Commented:
Thanks for the quick answers, guys.

The former domain is going away completely.  Users will be newly created.  Only a few computers will be moved to my domain.  The only thing we'll be keeping is any data in the form of documents.
Radhakrishnan RSenior Technical LeadCommented:
Ok..then demote / promote the DC.
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