SQL & PL/SQL :: Converting Number Column To Date Column

I have a partitioned table with ~50 million rows that was setup with a number(10) instead of a date column. All the data in the table is ALWATS in this format YYYYMMDD

CREATE TABLE T1.monthly
(
SEQ_NUM NUMBER(10) NOT NULL,
DAY_DK NUMBER(10) NOT NULL
)
TABLESPACE USERS
PCTUSED 0
PCTFREE 10
[code]........

some sample data

SEQ_NUM DAY_DK
---------- ----------
990 20121225
991 20121225
992 20121225
993 20121225
994 20121225
995 20121225
996 20121225
997 20121225
998 20121225
999 20121225

When I use the exchange partition method the parition is able to move the data from "monthly" table to "mth" table.

desc t1.mth; ### my temorary table
Name Null? Type
----------------------------------------- -------- ----------------------------
SEQ_NUM NUMBER(10)
DAY_DK NUMBER(10)

Than when I try to alter my temp table "mth". I get an error table must be empty to change column types.

alter table n546830.mth modify (DAY_DK date);

Next I tried making my temporary table "mth" a date column. When I an the exchange partition command I get the following error:

alter table t1.monthly exchange partition DEC_2012
with table t1.mth without validation;
alter table n546830.monthly exchange partition DEC_2012 with table n546830.mth without validation
*
ERROR at line 1:
ORA-14097: column type or size mismatch in ALTER TABLE EXCHANGE PARTITION

Method I can use to convert a number(10) to date column and keep the information in a table. Note, I don't care about HH:MM:SS as I never had that information to begin with and would be happy to set that part of the date column to all zeroes "00:00:00"