changing Local Domain Name

Hello,

What is the specific reason of this change ? To be honest, there is no reason that you should change you internal domain since, if you need to proceed, you'll have to rebuild a complete Forest.

It will affect all of your environment.

I really don't recommend it.

to deterrent company login to the same DC.
now this company split.
the users for company A want to see in the login there name or something else but not the other company name (also legally)

And do the users from Company A need to have access to other company ressources ?

If you just need another UPN suffix, you can simply add this within Active Directory Domains and Trusts > right-click on the root (Active Directory Domains and Trusts) > Properties and then add the new UPN suffix (see the screenshot attached).

Change the UPN suffix for all the users concerned (add this new company in a new OU), and apply this change to all the users concerned within this OU (with PowerShell if there is a lot of users).

Then users will be able to connect through username@testabc.local.

For the UPN change :

Get-ADUser -Filter * -SearchBase 'ou=nonAdmins,ou=User,ou=Administration,dc=do,dc=local' -Properties userPrincipalName | foreach { Set-ADUser $_ -UserPrincipalName "$($_.samaccountname)@testabc.local"}

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You'll have to change the part in with the OU and the new UPN where @testabc.local is located.

just to clearly:
all users are connected to the same data center
the only important thing is: to split the logon domain name
and to have more flax ability with gpo.