No, an executor may NOT change the will--no one can, other than the testator (person who made the will). The executor must follow the will as written, regardless of what she wants to do. Your grandfather can change the will--it is his will--but if he makes changes when he has dementia, they could potentially be challenged (such as by an heir who was going to receive more under the original version of the will) on the grounds of lack of mental capacity, since only a person who is mentally competent can make or change a will. Such a challenge would have to be made after he is dead--a will has no effect until death and so any challenge to it pre-death would be moot; and also, you have no right to see a person's will while he is alive--and would require medical testimony (e.g. from a doctor who had been treating him at the time) and other credible evidence to show that he was mentally incompetent when he changed the will.