ALTER TABLE my_table MODIFY (my_column new_datatype);
You should issue the command when no one is using the table as it will lock users from modifying the table's contents until the command completes. In order for the above command to succeed, the data in the column must not violate the new datatype. For instance, you will not be able to convert from VARCHAR2 to NUMBER unless the column only contains numbers in character form. If there are characters, the above will generate an error.
Your DBA should be willing to help you with this, even if it is your responsiblity to write the script. If your DBA is not willing to help you, then I would recommend talking with management as they may not be doing thier job.