We have an on premise exchange 2013 in our domain, and recently acquired a company that uses a hosted, Exchange Online (Office 365). The plan is to migrate their users into our domain, and create a hybrid Exchange environment. From what I've read, this would involved using the Hybrid function in Exchange 2013 to add the Office 365 account, being sure to add our domain to the Exchange online as well. The assumption here is that this is the first step in this process, and that migrating users from their domain to ours is the next step, and then perform any necessary mailbox migrations.
Is this the proper order for this type of migration/merge? My concern is disconnecting mailboxes and losing users email. We want them to be able to log into our domain, and still maintain access to their email, whether moved to the on premise or still residing on the Exchange online.
I've migrated domains and exchange environments before, but this one has a slightly more complicated layer to, especially because I've never used Exchange Online before. Any guidance on this is greatly appreciated!