There are several different scenarios that might help you determine what type of volumes you need. For example, if you wanted persistent names or to make full use of EVMS features, such as Drive Linking or Snapshotting, you would convert your compatibility volumes to EVMS volumes. In another situation, you might decide that a volume needs to be read by a system that understands the underlying volume management scheme. In this case, you would convert your EVMS volume to a compatibility volume.
A volume can only be converted when it is offline. This means the volume must be unmounted and otherwise not in use. The volume must be unmounted because the conversion operation changes both the name and the device number of the volume. Once the volume is converted, you can remount it using its new name.