Changing date to sort code format?

Format the cells Number format > Custom
Type

Formula:

dd-mm-yy

Or if you require a string returned

Formula:

=IF(A2="","",TEXT(A2,"dd-mm-yy"))

Find and replace will not work because the displayed date is the serial date formatted "dd/mm/yy"
i.e.
29/09/2012 is actually 41181 so there are no "/"s to replace.