Adding a new e-mail domain to a postoffice

My company is undergoing a corporate reorganization as of July 1
Hubbard Realty will become Hubbard Commercial. This means that our
e-mail address will be changing from HubbardRealty.com to
HubbardCommercial.com what I need to know from you is there a way to set
up the new HubbardCommercial.com as the primary e-mail account with each
of my users but still allow them to keep all their old HubbardRealty.com
e-mail records and if someone sends an e-mail to HubbardRealty.com. It
would forward to the HubbardCommercial.com account.