In theory, yes: people do suffer neurological, chemical (e.g. substantice abuse), pyschological, etc. conditions which impair their cognitive ability and render them incompetent, but which then  improve, leaving them mentally competent again. If that happens, they regain their legal competence, too--the law does not make a determination of incompetence an irrevocable finding, but rather allows it to be modified based on the person's condition.  This is very unusual in the case of Alzheimer's, since to my knowledge, while there are better days and worse days, it does not overall improve; that is why I started this answer by writing "in theory," since it does not seem like that would happen with an Alzheimer's-based diagnosis.What makes it at least somewhat plausible, however, is that your father had an auto accident. Physical trauma, like being shaken up or having your head hit something (or being hit by an airbag) in an accident can cause temporary mental impairment which then gets better as swelling/bruising subsides, or as certain damaged pathways are re-routed around. If the incapacity was due at least in part to the accident, even if it had at the time been called Alzheimer's, it is again at least plausible that this has occured.However, if you mistrust the doctor or his diagnosis, speak to an elder law attorney about how you might challenge this.