
poj84 (poj84) asked a question.
Active Directory's ObjectGuid is hard matched to Okta's ExternalID. When a user is moved to a new forest they receive a new ObjectGuid and those 2 attributes no longer match. Both are immutable. How can that match be established again in large enterprise environments where uptime is crucial?

so support had no answer for me last week(case still open)
I found this https://support.okta.com/help/s/article/After-adding-a-second-AD-instance-provisioning-issues-for-Office-365-are-encountered
was ok answer but with the info made a better answer
what I did was created Okta attributes for both domain
Domain-a-immutableID
domain-b-immutableid
then wrote expression that is legacy ID exist use it if not use new domain.
user.domain-a-immutableID != null ? domain-a-immutableID : user.domain-b-imutableid
(this field is at create only)
so now user get legacy domain unless it does not exist then they get new domain.
there is also a powershell to update users Immutable id in O365 so I will get to a migration point where most are in new domain... I will export ID from Okta and run a script to mass update O365 to new domain ID then remove the legacy domain mapping.