is_unique in Form Validation on UPDATE

If you want to do this, just run a search for the slug and count results in your own validation routine. Then you can exclude the current record easily. However you should not really be changing the page URL without the user agreeing to it, or even being in control of it. Why not add the slug to the form and put an instruction/warning that changing it will change the page url. Also, you can make sure the column is defined as unique and catch the error when updating.