Hi,
It is possible to migrate some or all of the mailboxes in Exchange online in a hybrid environment. The move of the mailbox to Exchange online can be done without loss of emails and side impact on the client side. But you should restore the connection and the synchronization between your on-promise Active directory and Azure active directory.
Ways to migrate multiple email accounts to Microsoft 365 or Office 365
Once you still use your on-promise active directory for authentication and GPO, you should keep Azure AD connect. In hybrid infrastructure , Azure AD connect is required to ensure the Single Sign on and the synchronization between Azure active directory and your local active directory.
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