Changing password after it has expired

Somebody on this list will know the definitive answer(s) to this question. I have been knocking holes in the wall with my head all day and cannot get an answer that makes sense.

In active directory you can set a password as expired and when the user logs in they get to type their old password to prove they are who they say they are and then new passwords to get the change to happen.

I want to achieve this via the LDAP interface but cannot find any references that say if it is possible. I suspect that what really happens under the cover is that the 'LDAP' code checks that the hash of the presented old password matches the value in the AD and then uses a privileged account rather  than the user to do the actual change (I am thinking of the IISADMPWD application here!) What I had hoped I could find would be an options that would allow a bind to succeed using the users credentials (old password/username) that could only change the password. But I have not.

Am I right in that this is done by knowing that the HASH matches or is there a hidden control to the AD LDAP interface I am missing?