What i understood is that there will be still the old OnPremise Stuff with the SQL Server and an Classic ADDS Service, right?
There is a Tenant with a Azure AD that will be managed by another Provider in the Future, right?
Or will there be a new Tenant with a new Azure AD and all Resources will be moved to the new Tenant?
The Azure AD of the Tenant is filled with what Identity's? The ones from the existing OnPremise Domain or Cloud Only Users or maybe a totally other Domain?
The change of Control of the Azure Tenant should be no Problem at all. The old OnPremise stuff could stay untouched.
Very important is communication of all involved partys, so the OnPremise Provider and the Azure Tenant Admin Provider, as they are connected in the direction of Network and Identitys.
Hope this was a bit helpful.
Kind Regards, Peter