How to plan an Active Directory domain rename without it killing me?
If your organization changes and you need an entirely new directory structure, sure take the opportunity to pick a best practice DNS name. But you have not identified a problem, either technical or user experience, worth doing a rename project.
Adding a UPN of COMPANYNAME.COM or perhaps COMPANYNAME and doing a UserPrincipalName conversion, is supposed to be easy. Describe this to users as logging in with (what looks like) their email address. Although, you trained them to separate email credentials from AD DS, so this may be confusing.
ACRO.TLD in an internal network security zone is fine, you can keep that. Register the name, just in case clients bypass internal DNS. Challenges come if users expect something else, or expect this to be the public presence (web server).
ACRO.COMPANYNAME.COM is already a hostname registered in Internet DNS.
I suggest avoiding the public presence names, even if you can design around the conflicts and confusion. Perhaps something like ACRO.COMPANYNAME.NET.