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There are no facilities for LDAP writebacks outside of the managed domain in that virtual network, which means that the changes are NOT written back to the on-prem AD through the AD Connect sync process if you have configured in this case you dont have anyways.
Only option I can think of is setting up the new domain and migrate the resouces unless you want to check the pricing and work on that, Azure Active Directory Domain Services usage is charged per hour, based on the SKU selected during the deployment process.
Check the pricing and see if reducing the SKU might help ? active-directory-ds