The issue is not the diagnosis per se: it's whether at the time of the change, your mother was mentally competent or not. (You can have Alzheimer's and, especially in the early stages, still be mentally competent). If your mother was competent, the changes are valid; if not, they are invalid, because a mentally incompetent person cannot enter into, create, change, etc. legal documents. If you believe your mother was not competent at the time and want to challenge the POA, you'd need to bring a legal action challenging it, in which you'd have to prove in court, by a "preponderance of the evidence" (i.e. that it was more likely than not that your mother was then incompetent), using medical testimony and evidence which shows that your mother was incompetent at the time she changed the POA. This may be difficult to do. If you want to explore this option, consult with an elder law attorney.