How to migrate to one domain name?

Domain trust or joining the domain to the parent domain name?

We have taken over a big company with a lot of big and small sites all arround the world. We had already setup a trust between the domains but now we want one single domain name,so everybody is logging into the same domain for the ease of use for sharing everything.

The quistion is where do i start, are there any roadmaps with steps to follow?
We are from the old timeline so we run everything on-premise.
Lets assume we are on the level of 2012R2 domain and forest. ( we do have a couple of 2008r2 but will be fasing out soon)
If 2016 is a better and more easy to follow plan then we are able to do first an upgrade to Domain and forest 2016.

I always worked in a single domain so all information is welcome at this time.
Many thanks,
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Dr. KlahnPrincipal Software EngineerCommented:
From a marketing viewpoint, consolidating all the domain names into one is throwing the goodwill for each lost domain out the door.

The value of "goodwill" is hard to determine, but personally -- when a company I have done business with for years is assimilated into a larger company and that identity disappears, I start shopping the market again from scratch to see if anybody has a better deal.
Aaron GuilmetteTechnology Solutions ProfessionalCommented:
When you say "migrate to one domain name," what exactly are you talking about?  The actual user accounts being consolidated into a contiguous Active Directory forest? Or migrating email?

If it's authentication infrastructure and consolidation into an existing domain structure, you can use ADMT to migrate users, groups, and computers in.

If we're talking about messaging, you can configure a trust and perform cross-forest mailbox migrations (ending up with linked mailboxes).

You can also use third-party tools to migrate mailboxes in.

If it's both, you can perform both activities.  As previously mentioned, I'd recommend preserving the previous email domains in the target environment for both marketing/business recognition as well as replay ability to old email.  You can set the reply email address to a new domain and keep the old domain email addresses as proxies.
Shaun VermaakTechnical Specialist IVCommented:

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ottenshxAuthor Commented:
I can see i can only pick a best solution so i did so . But all anwers where helpfull , many thanks.
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