mySQL convert varchar to date
select date_format(str_to_date('31/12/2010', '%d/%m/%Y'), '%Y%m');
or
select date_format(str_to_date('12/31/2011', '%m/%d/%Y'), '%Y%m');
hard to tell from your example
As gratitude to the timely help I got from here - a minor update to above.
$query = "UPDATE `db`.`table` SET `fieldname`= str_to_date( fieldname, '%d/%m/%Y')";