Azure AD Connect Merge Account

Hello everyone, I have an issue with one of my clients. Here's the situation. Our client has a local Exchange server with mailbox users. They also have AD users different from mailbox users. So the customer synced with Azure AD connects its Exchange users to O365. Ex: user1_Exchange. Local authentication in Windows is done with user1 and Outlook uses user1_Exchange. He migrated all his mailboxes to Exchange Online. Now it would like to merge its AD users with its O365 users. Example: User already sync: user1@example.com Now we want to sync or merge 1user@example.local to user1. We can't just delete accounts in O365 because mailboxes are already migrated. We have a hundred users. Does anyone have an idea? Thanks