The only option that you have is to delete and re-create the ADDS in another subscription.
Can I move an existing Azure AD Domain Services managed domain to a different subscription, resource group, region, or virtual network?
No. After you create an Azure AD Domain Services managed domain, you can't then move the instance to a different resource group, virtual network, subscription, etc. Take care to select the most appropriate subscription, resource group, region, and virtual network when you deploy the Azure AD DS instance.