Now they are ready for synchronization with their on premise infrastructure but don't want duplicate ids to be created so here are the summary of steps that needs to be done so that user accounts are merged.
- ADD a domain to office365 installation by following ADD domain wizard, example lab.com.
Go to office 365 admin console --> Settings --> Domains --> ADD Domain
This will ask you to create a TXT record in the DNS for lab.com
Value: MS=code for authorization
TTL: 3600 or provider default
This record assists Microsoft to check the authorization of the domain, this will not change any of the services that are already running in the environment so just create it.
Note: First You have to change the on-premise users UPN to Primary SMTP address, before synchronization of Active Directory to the cloud.
Ones the domain is added you need to change the UPN of Cloud user to match the UPN of on-premise user i.e. Primary SMTP address.
As UPN matches on both locations so ones you synchronize AD by using Azure AD connect display Sync Type will change from In cloud to synced with Active directory as shown below.