Update field with new string made from characters in existing value

Question

field is meta_value and current value is like 01/01/2006

i want to change it to a string 20060101 so thought i could get the value and update it with the characters in the correct order.

ie

UPDATE wp_postmeta SET meta_value = meta_value[9]meta_value[8]meta_value[7]meta_value[6]