You have to deploy or configure Entra connect server to sync new domain to Entra ID.
Regarding the match between AD account and Entra ID ( Azure) account , there are two method. Soft match , based on mail and UPN. SO if you keep the same UPN or mail you soft match can relink new AD account to old Entra ID account. Hard-Match based on msds-consystencyGUI attribut in AD and Immutable attribut in Entra ID. I invite you to read this article for more details : Microsoft Entra Connect: When you have an existing tenant
If the both method is not working or complicated to implement, you can delete all User account in Entra ID and let Entra connect server or Cloud Sync in order to sync all users account in the new domain by creating new account in Entra ID. But in this case user risk loose access on his data in Azure
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