Updating domain of a Hybrid Azure AD User account

Please refer to the best industry recommended practices mentioned below,

Important:

• The defined scenario is hybrid in nature and unfortunately, we do not have any Microsoft documentation to achieve the desired results and had to adopt a custom approach
• The options shared below are best-effort suggestions and needs to be planned and implemented on your own responsibility with a complete understanding
• Back up all the user data (.pst file, one Drive data, etc.) so that no data is lost as a result of any confusion or misunderstanding

Option 1: To keep on using the same Hybrid (Synced from local AD) User account 

• Make sure the new domain is added to your AD environment, if not add it. 
• Update the domain to the desired user.  
• Use the "ProxyAddress" attribute in the Attribute editor. 
• Use SMTP for the new primary email address
• Use smtp for the old email address and any other additional email address that has to be an alias. 
• Save the changes and sync your directory 

Option 2: Creating a new shared mailbox

• Create a new shared mailbox with the new domain 
• Assign Read and manage plus Send As permission to the referred user on the new mailbox

Option 3: Converting hybrid Account to Cloud-Only (Not recommended)