I need to combine two different office 365 tenants after a merger or acquisition. Our company X (250 users) has acquired another company Y (200 users), both companies have their on-premise active directories and both companies have already enabled Azure AD Connect to sync their AD users with Office 365 tenants for emails that means emails of both companies are in office 365 but in their own tenants. Using only mailboxes, no onedrive/sharepoint.
After google, found out no simple way to combine/migrate one tenant to another tenant and for this I would need to use Hybrid approach on either side ( would preffer on company Y side) for this migration. At the end, email addresses of company Y e.g Y.com will become X.com as primary and Y.com will be added as alias, main concern is to avoid re-configuring of outlook profiles of company Y users when their email domain will be changed to X.com in X tenant and also want to avoid computer domain login issues. Never done such migration before so looking for directions, can someone please provide guidance?
As for as I know, would need to move mailboxes downwords from 365 to one-premise exchange in Hybrid on Y side, cut off from 365 tenant then setup hybrid again with X tenant, right? what to do next? please advise.
Thank you in advance.