Unplug the existing Pluggable Database in 12c.

Before unplugging lets check the Container databases status.

SQL> show con_name

CON_NAME
------------------------------
CDB$ROOT

SQL> select NAME,CON_NAME,CON_ID from v$active_services;

NAME                 CON_NAME                           CON_ID
-------------------- ------------------------------ ----------
pdbtest.oracle.com   PDBTEST                                 4
pdborcl.oracle.com   PDBORCL                                 3
cdb12cXDB            CDB$ROOT                                1
cdb12c.oracle.com    CDB$ROOT                                1
SYS$BACKGROUND       CDB$ROOT                                1
SYS$USERS            CDB$ROOT                                1

SQL> select CON_ID,NAME,OPEN_MODE from v$pdbs;

    CON_ID NAME                 OPEN_MODE
---------- -------------------- ----------
         2 PDB$SEED             READ ONLY
         3 PDBORCL              READ WRITE
         4 PDBTEST              READ WRITE


SQL> conn /as sysdba
Connected.
SQL> alter pluggable database pdborcl close immediate;

Pluggable database altered.

SQL> alter pluggable database pdborcl unplug into '/u01/app/oracle/oradata/cdb12c/pdborcl/pdborcl.xml';

Pluggable database altered.

SQL> drop pluggable database pdborcl keep datafiles;

Pluggable database dropped.

SQL> select CON_ID,NAME,OPEN_MODE from v$pdbs;

    CON_ID NAME                 OPEN_MODE
---------- -------------------- ----------
         2 PDB$SEED             READ ONLY
         4 PDBTEST              READ WRITE

SQL> select pdb_name, status from cdb_pdbs;

PDB_NAME        STATUS
--------------- -------------
PDB$SEED        NORMAL
PDBTEST         NORMAL

SQL> select NAME,CON_NAME,CON_ID from v$active_services;

NAME                 CON_NAME                           CON_ID
-------------------- ------------------------------ ----------
pdbtest.oracle.com   PDBTEST                                 4
cdb12cXDB            CDB$ROOT                                1
cdb12c.oracle.com    CDB$ROOT                                1
SYS$BACKGROUND       CDB$ROOT                                1
SYS$USERS            CDB$ROOT                                1

Hope this help.