Symptom:
You receive the following error when trying to publish your new Lync Topology, after removing the last Lync 2010 Front End Server.

ConferencingPoolInUse: The new topology XML removes services that will orphan existing conference directories.

Error: Cannot publish topology changes. Conference directories still exist on a pool that would be deleted. Remove the conference directories before continuing.
▼ Details
└ Type: InvalidOperationException
└ ▼ Stack Trace
└ at Microsoft.Rtc.Management.Deployment.Tasks.PublishTopologyTask.VerifyOrphanedDirectories()
at Microsoft.Rtc.Management.Internal.Utilities.LogWriter.InvokeAndLog(Action action)

Error: An error occurred: "System.InvalidOperationException" "Cannot publish topology changes. Conference directories still exist on a pool that would be deleted. Remove the conference directories before continuing."

Solution:
On one of your front end lync servers, open up the Lync Server Management Shell as an Administrator.  Once open, execute the following command:

Get-CsConferenceDirectory

For each of the items that have a ServiceID referencing the Front End server you are trying to retire, execute the following command (replacing the appropriate values with the output found from the previous command):

Move-CsConferenceDirectory -Identity X -TargetPool poolorfrontendserver.mydomain.local

After all conferences have been moved off of your Lync 2010 Front End server, you should be able to successfully publish the topology.