We currently have 2 Exchange 2010 SP3 on prem servers with all roles installed on each for company1.org. We also have a tenant on Office 365 with company1.org registered as a domain. We are syncing some of our OUs from our on prem AD with Office 365 using AD connect. Some of our users were in Office 365 before we began to sync the directories. However, we are able to get them to sync with a soft join. We would like to create a hybrid deployment to begin moving our mailboxes to Office 365 but I would like to know how to merge the mailboxes that exist in Office 365 with the active mailboxes we will be moving from the on prem exchange servers. Is it possible to merge? Should we turn off mail in Office 365? What would be the best method to achieve our goal. After the mailboxes are moved and all is working well we will decommission our on prem servers.
Thank you,