After you finish migrating all your mailboxes, you will turn off directory synchronization, and then you can decommission ...

After you finish migrating all your mailboxes, you will turn off directory synchronization, and then you can decommission your on-premises %BRAND_EXCHANGE_ONLINE_SHORT% servers. Note that after you turn off directory synchronization, the passwords for your users' on-premises accounts and their %BRAND_OFFICE_365_SHORT% mailboxes will be different. To learn more, see Exchange Server Hybrid Deployments.