You need to ensure that the stale resource records are automatically removed from the contoso.com zone
You need to ensure that the host (A) records for the removed computers are automatically deleted from the contoso.com DNS zone
You need to ensure that users in contoso.com can connect to servers in sales.contoso.com by using fully qualified domain names (FQDNs)
You need to identify if all of the DNS records usedfor Active Directory replication are correctly registered
You need to ensure that the new zone is replicated to the domain controllers in only one of the sites
You need to ensure that the custom application directory partition replicates to the new domain controller