How to make partitioning in existing mysql table?

he answer which is very simple, use to_days function

ALTER TABLE tbl_rtdata  PARTITION BY RANGE (Month(fld_date))
(   
PARTITION p_Apr VALUES LESS THAN (TO_DAYS('2012-05-01')),
PARTITION p_May VALUES LESS THAN (TO_DAYS('2012-06-01')), 
PARTITION p_Jun VALUES LESS THAN (TO_DAYS('2012-07-01')),  
PARTITION p_Jul VALUES LESS THAN (TO_DAYS('2012-08-01')),
PARTITION p_Aug VALUES LESS THAN (TO_DAYS('2012-09-01')),
PARTITION p_Sep VALUES LESS THAN (TO_DAYS('2012-10-01')),
PARTITION p_Oct VALUES LESS THAN (TO_DAYS('2012-11-01')),  
PARTITION p_Nov VALUES LESS THAN (TO_DAYS('2012-12-01')),
PARTITION p_Dec VALUES LESS THAN MAXVALUE );