Hi RGlide,
As I understand it, your old domain is abc.com, and the new domain is xyz.com. The users will use the new xyz.com account in Office 365 and you want the emails send to abc.com can be forwarded to xyz.com.
If so, in Office 365, you can use E-Mail Migration in the Exchange Control Panel to migrate mail from an on-premises messaging system to your cloud-based organization. Exchange Online provides Cutover Exchange migration, Staged Exchange migration and IMAP e-mail migration. You can choose the migration type according to your exchange environment.
For more detailed information, please refer to the link below:
http://help.outlook.com/en-us/140/ms.exch.ecp.emailmigrationstatuslearnmore.aspx
After migrate to Office 365, you can set up a forward rule which uses to forward emails to xyz.com.
Thanks,
Eric Sun