Change month partition to day partition with current data in Oracle 12c
The above affects only partitions created in the future.

If you want to change your monthly partitions into daily partitions, you can use:

ALTER TABLE ... SPLIT PARTITION ...

Example:

ALTER TABLE sales SPLIT PARTITION sales_201701 INTO
( PARTITION sales_20170101 VALUES LESS THAN (TO_DATE('02-JAN-2017','dd-MON-yyyy')),
  PARTITION sales_20170102 VALUES LESS THAN (TO_DATE('03-JAN-2017','dd-MON-yyyy')),
  PARTITION sales_20170102 VALUES LESS THAN (TO_DATE('03-JAN-2017','dd-MON-yyyy')),
  PARTITION sales_20170103 VALUES LESS THAN (TO_DATE('04-JAN-2017','dd-MON-yyyy')),
...
  PARTITION sales_...);


More examples: https://oracle-base.com/articles/12c/online-split-partition-and-subpartition-12cr2