I ran into trouble when I switched the selected Time Machine volume on the three clients, getting the dreaded "<machine>.sparsebundle is already in use" error with the asinine "try ejecting the server" recommendation. The volumes seemed to be mounting and unmounting normally and the clients had the appropriate credentials. The one thing that didn't look right was that the listed owner for the individual backups in the Time Machine server interface was "Unknown Owner." After a long and fruitless session of Google searches (most troubleshooting information is about Time Capsule, not Server) I finally came across this site about moving sparsebundles from a Time Capsule to Server—a process not unlike moving a sparsebundle from one server to another. Following the second method listed there—crucially the recursive chown on the sparsebundle image to the username on the Time Machine Server— I was able to correct the Unknown User to the correct one, with smooth sailing thereafter.
Other have pointed out that this ".sparsebundle already in use" error can result from a wide variety of different problems pertaining to AFP on both the server and the client. The above makes it seem yet worse—ownership and permissions issues can also result in the same error message.