Try this:
ALTER TABLE table_name CHANGE old_column_name new_column_name INTEGER;
This assumes your old column data type is an integer. If your definition is different than the orgininal column type MySQL with try and convert the data to the new type. Just make sure you specify the same definition as the old column data type so it doesn't touch your data.
You can read more about this in the MySQL manual here if you need more help.
http://www.mysql.com/documentation/mysql/full
Troy