Datatype conversion and Range

Question

Ok here i am back again..
I am creating a database upgrade tool,
the column types are also subject to change.
I use the CAST(col_name AS newtype) function to cast types,
this often works. But not for all cases.
@see http://download-west.oracle.com/docs/cd/A91202_01/901_doc/server.901/a90125/functions15.htm
For example a change from VARCHAR(255) to CHAR(3) does not work if the row currently
contains a row with more than 3 character.
A simple conversion that solves this problem is:
CAST(CAST(col_name AS VARCHAR(3)) AS CHAR(3))
I tested it using the following statement:
select CAST(CAST('wtfomgbbq' AS VARCHAR(3)) AS CHAR(3)) from dual;
Does this always work?
Some type changes can be done directly on a table like this:
ALTER TABLE table_name MODIFY ( col_name <NEWTYPE>)
But this only works if the range is either the same or increased,
or the table does not contain any rows, or the columns has only null values.
(i checked the reference)
But is there a way to always use this MODIFY statement and force the data to be transformed to the new type?
My current approach is to create a temporary table, drop the current table, and recreate the current (with correct schema) and put the old data in the new table using the CAST function as described above.
Maybe you experts now some way to do this faster?
And how can i handle range decreases without getting oracle errors?
(the update process may take several hours, so the DBA does not want an error message when updating the database)

Answer 1

It would be easier as you still have to convert the output of SUBSTR to CHAR(3), else you are doing an implicit type conversion... Which is not bad, but i really want to be explicit:
CAST(SUBSTR(col1, 1, 3) AS CHAR(3))
Note that the sql code that does the type conversions is automatically generated, so it doesnt matter if it looks more complex.
Anyway the main problem question is:
Can i always use ALTER TABLE MODIFY to modify the data types of columns or do i have to use temporary cols, or temporary tables?
Isnt there an option to force data type conversions and let oracle handle the casting (ans loss of data if the data-range is decreased) ?
Thanks in advance