When I had to edit an option in the "Edit Custom Field Options" (ex.: an option that is showed in a combo box) that is already associated in any issue, I have to add a new option, then find all the issues associated with the older option, alter the option selected (to reflect the new option) then go to "Edit Custom Field Options" and remove the older option. It´s very expensive to do and I think it´s going to be easy if we have an "Edit" option in the table of "Edit Custom Field Options".