You want to enforce the autonomy of the previous stage of the person, when the person actually changed. What I want to say is that we always change all the time. When we are competent, we change, let alone when we don't know or understand the brain enough to make a decision about the patient now when he is in advanced dementia.
I say all the time that at any stage, the issue of advance directives for dementia patients is morally problematic; therefore, this is where I draw the line. I've nothing against advance directives when it comes to competent, autonomous patients.