Email address consolidation

In my scenario we have four branch offices and head office with different domains and different email address as below

User1@ Branchoffice1.com> using Exchange 2010
User2@ Branchoffice2.com > using Exchange 2010
User3@ Branchoffice3.com >using Exchange 2013
User4@ Branchoffice4.com> using Exchange 2010
User5@Headoffice.com>     using Exchange 2010

According to management decision, I need to work to consolidate  email address for branch office, meaning that all branch office users will get headoffice.com email address so that they can use both internal and external email address as headoffice.com instead of branchoffice.com.

At present, management do not want to move mailbox from branch office to head office rather only email flow so that all branch office users will send and receive email via headoffice.com sitting in their own email infrastructure.

Branch office users will be using branch office exhange sever for their active directory and exchange authentication but their email address will be headoffice.com instead of branch office.com. and we are not planning to move branch office users mailbox to head office. Could you please advise.

Note: I have asked kind of similar question as the URL below whereby Peter and Simon gave feedback but now requirement is slight change for that repost it again.

http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/Q_28682687.html
ipsec600Asked:
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Simon Butler (Sembee)ConsultantCommented:
You are sharing the SMTP address space.
This is going to get complicated very quickly.

All users will need to have accounts on the primary server.
These accounts will then need to forward the email to the actual server. That will mean leaving the old email addresses active so Exchange can forward the email to the correct place. All users will have two email addresses - the main domain set as their primary and the old domain set as the secondary.

You will then need to configure each branch office server to route email for the main domain to the main server.
https://technet.microsoft.com/en-us/library/bb676395(v=exchg.141).aspx

If they want all users to be seen in the GAL, then that will get difficult to manage as you will need to have all users with accounts/contacts on all servers. Can be done (and I have done it) but it means each new user would need to be created on all servers.

Simon.
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ipsec600Author Commented:
Thank you Simon for your excellent clarification.
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