It is a good idea to look at your will periodically to make sure that it contains the terms and conditions that you want at the time of your death. It's a good idea to review that will at least every five years or if there is a significant change in tax law you should certainly go back to your attorney and see if it's time to change your will.
Can you do it? Yes, but when you do, you have to be again, competent. And with Alzheimer's, dementia affecting many elderly people, sometimes it becomes too late to change a will simply because a parent is no longer competent. However, if a person is suffering from some physical or mental illness or disease, that doesn't mean they can't change their will but they have to be competent at the time they sign it.
That's why we use two witnesses plus a notary to make sure even though the person may have a disability, that the will that they signed on that given day and time is still legal and still valid, even though their disability may affect them before the signing and after the signing.