SQL> create table t(id number, val char(25));

Table created.

SQL> [insert statements]

SQL> select id
  2       , val
  3       , length(val) l
  4    from t
  5  /

        ID VAL                                L
---------- ------------------------- ----------
         1 ONE                               25
         2 TWO                               25
         3 THREE                             25
         4 FOUR                              25
         5 FIVE                              25
         6 SIX                               25
         7 SEVEN                             25
         8 EIGHT                             25
         9 NINE                              25
        10 TEN                               25

10 rows selected.

SQL> alter table t modify (val varchar2(25))
  2  /

Table altered.

SQL> update t set val = rtrim(val);

10 rows updated.

SQL> select id
  2       , val
  3       , length(val) l
  4    from t
  5  /

        ID VAL                                L
---------- ------------------------- ----------
         1 ONE                                3
         2 TWO                                3
         3 THREE                              5
         4 FOUR                               4
         5 FIVE                               4
         6 SIX                                3
         7 SEVEN                              5
         8 EIGHT                              5
         9 NINE                               4
        10 TEN                                3

10 rows selected.