User can edit/delete their custom titles only if they are allowed to do so. Under Member Management -> Profiles you can set which groups have access to editing their own custom titles.
If you want to reset your current post-based titles overriding the old custom titles, then you can check the checkbox next to "Check this box if you want to update existing users with these new titles." on that same page. This can take awhile however as it has to update each one of your users.
