Can a person in the last stages of Alzheimer’s legally change a Will?

Wills are generally contested either of 2 grounds. The first is that the testator (i.e. the person making the Will) lacked "testamentary capacity"; this means that they were incompetent to make (change) a Will at the time of signing it. The second ground is that because of the testator's mental state, they were subject to "undue influence"; this means there was evidence of coercion, manipulation, deception, compulsion, intimidation, etc.

In the case of Alzheimer's, in the early stages a testator may have the requisite capacity to understand, know or recognize that a Will is being made; the extent of their estate; who the beneficiaries are; and the manner in which their will be distributed (e.g. what will be kept in the family or sold). They could also have had the ability to make free choices in distribution of the estate, thereby negating the argument of having been unduly influenced in the making of their Will.

That having been said, someone who has been medically determined to have been in the last stages of Alzheimer's would be deemed to have lacked the requisite testamentary capacity to have executed a Will and/or the Will could be challenged as to just how free their choice was in determining beneficiaries and what their share of the estate would be, etc. In fact, any time a person has Alzheimer's and executes a Will, there is always the potential for a Will contest even if the testator was in the early stages of the disease and the time of signing.