From the docs... once set it cannot change.
[-prefix <text>] - Snapshot Copy Name Prefix for Schedule
This option specifies the prefix with which Snapshot copies will be created for the added
schedule. Every schedule has only one prefix. Once a prefix gets associated with a
schedule, you cannot update the prefix. If some prefix is already associated with the
schedule and you do not specify this parameter, then the previously defined prefix
is used. The command fails if you try to update an existing prefix for a schedule. If
no prefix is associated with the schedule and you do not specify this parameter, then
schedule name is be used as the prefix.
