The System Management Guide states "You cannot change a member from PRODuction to TEST", but here is a method to accomplish your goal.
The members can be copied as new members in the same Panlib.
The production members that were copied should be renamed.
The members created from the copy processing are automatically placed in TAE (test) status so changes will be allowed.
The members that were renamed (currently in production status) can be deleted by executing PAN#2 ++DELETE.
To automate the process, use the PGP (Panvalet Group Processor). This processing generates the Panvalet commands automatically, placing them in a SYSPUNCH file that is read and processed in a subsequent step.