User migration from one domain to another domain with mailboxes

Dear All,

In my client we are using AD 2k8 R2 (FFL and DFL 2003 Native) and exchange 2010. We have single forest where one parent Domain (corp.test.com) and one Child Domain (new.corp.test.com). Now my organization wants to merge Child Domain users to parent Domain. There are around 1000 users in child domain. After complete migration, they want complete remove the child domain.

Now I want to move all users with their mailboxes to parent domain.

If somebody has already done such activity, please suggest or share the plan.

Thanks in advance.