We have an AD Forest with two tree root domains. We will call them a.domain.com (Forest root domain) and b.domain.com. We have migrated all of the needed objects from b.domain.com to a.domain.com.  Now I want to decommission b.domain.com.  We have one DC left in b.domain.com.  Do I just need to run dcpromo and check the box in the wizard that states " Delete The Domain Because This Server Is The Last Domain Controller In The Domain"?

When I have removed b.domain.com will the users that were migrated to a.domain.com retain their UPN?  I.E. username@b.domain.com.

Thanks for any assistance you can provide!

"Do I just need to run dcpromo and check the box in the wizard that states " Delete The Domain Because This Server Is The Last Domain Controller In The Domain"?

Yes

"When I have removed b.domain.com will the users that were migrated to a.domain.com retain their UPN?  I.E. username@b.domain.com."

Do they have that as additional UPN now? If they do it should stay there

In addition, I will recommend before demotion the tree domain shutdown the server during business hours and check the impact if no issue is reported then you should be looking good to move ahead with demotion.

If the DC is last DC in the domain then you need to select "Delete The Domain Because This Server Is The Last Domain Controller In The Domain".
https://technet.microsoft.com/en-us/library/cc771839(v=ws.10).aspx#BKMK_remove_domain_interface

When the user is migrated to new domain the UPN will change to target domain name i.e the User logon name will be  Username@targetdomainname example in your case it will be Username@a.domain.com . If you want the UPN name to be added you can add the UPN to target domain.

Add User Principal Name Suffixes
http://technet.microsoft.com/en-us/library/cc772007.aspx

Users Can Log On Using User Name or User Principal Name
http://support.microsoft.com/kb/243280/EN-US