Dementia patients in the moderate-late stage of the disease can, and often do, express different preferences than they did at the onset of their condition. The received view in the philosophical literature argues that advance directives which prioritize the patient’s preferences at onset ought to be given decisive moral weight in medical decision-making. Clinical practice, on the other hand, favors giving moral weight to the preferences expressed by dementia patients after onset. The purpose of this article is to show that the received view in the philosophical literature is inadequate and is out of touch with real clinical practice. I argue that having dementia is a cognitive transformative experience and that preference changes which result from this are legitimate and ought to be given moral weight in medical decision-making. This argument ought to encourage us to reduce our confidence in the moral weight of advance directives for dementia patients.