How to work around ORA-12008 and ORA-01858 during an online redefinition

The following error occured during redefintion:

Move method (cons_use_pk or cons_use_rowid): cons_use_rowid
begin
*
ERROR at line 1:
ORA-12008: error in materialized view refresh path
ORA-01858: a non-numeric character was found where a numeric was expected
ORA-06512: at "SYS.DBMS_REDEFINITION", line 75
ORA-06512: at "SYS.DBMS_REDEFINITION", line 3459
ORA-06512: at line 2

The reason:
One of the reasons why I wanted to redefine the table, was to make it partitioned.
One of the column in the interim table was therefore defined as DATE instead of VARCHAR2:

Original table:

CREATE TABLE SCOTT.MYTABLE
(
  SEQ_NUM              NUMBER Generated as Identity ( START WITH 9984 MAXVALUE 9999999999999999999999999999 MINVALUE 1 NOCYCLE CACHE 20 NOORDER NOKEEP) CONSTRAINT SYS_C0073650 NOT NULL,
  ENTRY_ID             VARCHAR2(50 BYTE)            NULL,
  ENAME                VARCHAR2(100 BYTE)           NULL,
  CREATED_DT           TIMESTAMP(6)                 NULL,
  REVISION#            INTEGER                      NULL,
  ACTIVE_YEAR          INTEGER                      NULL,
  PERIOD               VARCHAR2(10 BYTE)            NULL,
  CONDITION            VARCHAR2(50 BYTE)            NULL,
  FN_ID                VARCHAR2(11 BYTE)            NULL
)
TABLESPACE USERS;

Interim table:

CREATE TABLE SCOTT.MYTABLE_INTERIM
(
  SEQ_NUM              NUMBER,
  ENTRY_ID             VARCHAR2(50 BYTE),
  ENAME                VARCHAR2(100 BYTE),
  CREATED_DT           TIMESTAMP(6),
  REVISION#            INTEGER,
  ACTIVE_YEAR          INTEGER,
  PERIOD               DATE,
  CONDITION            VARCHAR2(50 BYTE),
  FN_ID                VARCHAR2(11 BYTE)
)
PARTITION BY RANGE (PERIOD)
INTERVAL( NUMTOYMINTERVAL(1,'MONTH'))
(
  PARTITION P_INIT VALUES LESS THAN (TO_DATE('2015-01', 'YYYY-MM'))
)
TABLESPACE USERS
;

The requirement from the Developers was that the partitions should be range partitioned on dates in the format 'YYYY-MM'.

When starting the online redefintion process, I hit the error at the top in this post.

I started to search for the root cause, and executed the following:

select seq_num,TO_DATE(PERIOD,'YYYY-MM') from Scott.mytable
fetch first 5 rows only;

which resulted in

ORA-01858: a non-numeric character was found where a numeric was expected

It turned out that the following SQL would return the data I wanted, in the correct format, defined with the correct data type:

select seq_num,to_date(to_char(TO_DATE(PERIOD),'YYYY-MM'),'YYYY-MM')
from Scott.mytable
fetch first 5 rows only;

This Expression had to go into the redefinition-command, like this:

begin
DBMS_REDEFINITION.start_redef_table(uname=>'EVENT',
orig_table=>'MYTABLE',
int_table=>'MYTABLE_INTERIM',
col_mapping =>'SEQ_NUM SEQ_NUM,ENTRY_ID ENTRY_ID,ENAME ENAME,CREATED_DT CREATED_DT,REVISION# REVISION#,ACTIVE_YEAR ACTIVE_YEAR,to_date(to_char(TO_DATE(PERIOD),''YYYY-MM''),''YYYY-MM'') PERIOD,CONDITION CONDITION,FN_ID FN_ID',
options_flag=>dbms_redefinition.cons_use_pk);
end;
/

After this change, the redefinition executed successfully.