Migration AD domains

Hi to you all,

I'm a student and quite new to active directory. Right now I'm on an internship and I have to make a virtual project for my company.

I have to test the possibilities to "migrate 2 AD domains to one big 'top-level forest' with 2 subdomains which maintain their own identity and independency. When they have their own exchange environment, also migrate it to one environment.".

In the first phase we had to set up the 2 domains (alfa.be and bravo.be) with AD and Exchange. The domains have their own ip-addressing (DNS) and communicate with each other (Two way trust). I did this and I had no problems.

The description of the second phase is "In a second phase we expect the migration to a new top level forest where the 2 old domains are accomodated".


Now I'm stuck and I have a few questions:
- They want to keep the old domains, but is it still a migration then?
- What are the possibilities to do this?
I already found 2 solutions for this:
* Make a new OU in one of the existing domains and then migrate all users of the other domain to that OU.
* Make a new domain and migrate all users from the 2 domains to that domain.
But are there any more?
- Are there any other possibilites besides using admt?

I just need something to make a good start and then I can go on.

Thanks in advance!