The guideline of Modifying a Column

Guidelines :

• You can increase the width or precision of a numeric column.
• You can increase the width of numeric of character columns.
• You can decrease the width of a column if :
  - The column contains only null values
  - The table has no rows
  - The decrease in column width is not less than the existing values in that column
  
• You can change the data type if the column contains only null values.
  exception) CHAR to VARCHAR
• CHAR to VARCHAR2 , or VARCHAR2 to CHAR : Only if the column contains null values or if you don't change the size.