Hi @Logan Burt ,
Agree with above.
1.You could directly change the display name and primary emaild address of user mailbox in EAC in on-premises Exchange server.
2.You do not need to create a second tenant. According to the information you provide, you only need to modify the Display name and primary SMTP address for the user. After the change, the smtp address related to onmicrosoft.com of the mailbox will remain unchanged.
Below screenshots is a test in my lab environment, I changed display name from user3 to user3-1, and changed the primary email address from [email protected] to [email protected]. After AAD connect is synchronized, I can see that the primary SMTP address of the mailbox also has changed in Exchange online. After that, I can successfully send emails to this address, and the emails sent from the user's mailbox can also be successfully received by the recipient, and the displayed name and email address that I see are updated. 
In addition, please note the following points:
1.As mentioned above, please uncheck the “Automatically update email address…” before you change the primary SMTP address.
2.When the user logs in to the mailbox, the UPN address is used. It’s may be different from the primary SMTP address. 
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