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