When mailbox migrations are complete, run the 'Switch Mailbox' task from the Accounts tab from within ODM.
When complete, perform the cutover and domain move.
Here are links from our on-line help regarding the Domain Cutover.
This is a list of Search results in our documentation
https://support.quest.com/on-demand-migration/current/search-technical-documents?k=Domain+Cutover&cause=
This is a link to the On Demand Migration Current - Active Directory Domain Move Quick Start Guide
https://support.quest.com/technical-documents/on-demand-migration/current/active-directory-domain-move-quick-start-guide
If further assistance is required, an alternate resource is our Quest Professional Services team (PSO) Please contact your Quest Sales Account Manager or sales@quest.com as there are several resources from PSO, Pre-Sales, whom are experts in customization, design and implementation. They will be able to assist with this request.