Hi @A L
I think it's the right approach to start the migration to Exchange online before the AD migration.
If you want avoid the AD migration , you can add mydomain.com as additional UPN suffix.
I think it's better to keep .local for local domains to avoid any conflict with external DNS zones
Here are the major step that I recommend:
- Set up a hybrid environment by installing adconnect to ensure identity synchronization between Azure AD and the old domain. For your information ,the hybrid environment is required for Exchange online migration t.
- Migrate to Exchange online
- Once the migration to Exchange online is complete, decommission the on-premise Exchange servers.
- Promote the new domain mydomain.com
- Migrate AD objects to the new domain.
Take note , you should to keep the value of the mail attribute and MS-DS-ConsistensyGUID to be able to link the new user AD account to the Azure AD account generated from the old user account via adconnect
- Disable directory synchronization at adconnect level and wait 72 hours
- Add the a connector for new domain in adconnect and delete the old one o
- Enable ad connect synchronization . User account migrated to the new domain will be linked automatically to the same user Azure AD account and mailbox if you keep the same value of the mail attribute and ms-ds-consistencyGuid
For this type of migration, I recommend you to be accompanied by a specialist to help you in the different stages
Please don't forget to mark helpful answer as accepted