SQL> create table t (id int, ORDER_DATETIME date, FLAG_DATE DATE);

Table created.

SQL> col column_name format a26
SQL> col data_type format a15
SQL> select column_name, data_type, virtual_column, hidden_column
  2  from dba_tab_cols
  3  where owner='MICHEL' and table_name='T'
  4  order by column_id
  5  /
COLUMN_NAME                DATA_TYPE       VIR HID
-------------------------- --------------- --- ---
ID                         NUMBER          NO  NO
ORDER_DATETIME             DATE            NO  NO
FLAG_DATE                  DATE            NO  NO

3 rows selected.

SQL> alter table t rename column FLAG_DATE to FLAG_DATE_OLD;

Table altered.

SQL> select column_name, data_type, virtual_column, hidden_column
  2  from dba_tab_cols
  3  where owner='MICHEL' and table_name='T'
  4  order by column_id
  5  /
COLUMN_NAME                DATA_TYPE       VIR HID
-------------------------- --------------- --- ---
ID                         NUMBER          NO  NO
ORDER_DATETIME             DATE            NO  NO
FLAG_DATE_OLD              DATE            NO  NO

3 rows selected.

SQL> alter table t add (FLAG_DATE DATE GENERATED ALWAYS AS (TRUNC("ORDER_DATETIME")) VIRTUAL);

Table altered.

SQL> select column_name, data_type, virtual_column, hidden_column
  2  from dba_tab_cols
  3  where owner='MICHEL' and table_name='T'
  4  order by column_id
  5  /
COLUMN_NAME                DATA_TYPE       VIR HID
-------------------------- --------------- --- ---
ID                         NUMBER          NO  NO
ORDER_DATETIME             DATE            NO  NO
FLAG_DATE_OLD              DATE            NO  NO
FLAG_DATE                  DATE            YES NO

4 rows selected.

SQL> alter table t set unused column FLAG_DATE_OLD;

Table altered.

SQL> select column_name, data_type, virtual_column, hidden_column
  2  from dba_tab_cols
  3  where owner='MICHEL' and table_name='T'
  4  order by column_id
  5  /
COLUMN_NAME                DATA_TYPE       VIR HID
-------------------------- --------------- --- ---
ID                         NUMBER          NO  NO
ORDER_DATETIME             DATE            NO  NO
FLAG_DATE                  DATE            YES NO
SYS_C00003_21042822:00:07$ DATE            NO  YES

4 rows selected.

SQL> alter table t drop unused columns;

Table altered.

SQL> select column_name, data_type, virtual_column, hidden_column
  2  from dba_tab_cols
  3  where owner='MICHEL' and table_name='T'
  4  order by column_id
  5  /
COLUMN_NAME                DATA_TYPE       VIR HID
-------------------------- --------------- --- ---
ID                         NUMBER          NO  NO
ORDER_DATETIME             DATE            NO  NO
FLAG_DATE                  DATE            YES NO

3 rows selected.