Of course, you must deal with existing column
data. When modifying a tables column datatype you may want to
export the rows, redefine the table and then re-import you data.
In a traditional database you would need to
follow these steps to alter a column data type:
1 - Create the new column at the end of the table.
2 - Run an update to populate the new table column
3 - Drop the old table column
4 - Re-name the new column to the original column
name
Also, starting in Oracle 11g, the new
virtual column feature is useful for changing table column
datatypes.
