I think you would be a different you in a lot of ways, of course; but I truly believe you would still very much be you - the person who has loved the people you have loved, and the person who is still loved by those who have always loved you. People who have had loved ones with Alzheimer's Disease will generally tell us that they have "already lost" a very big part of the person they love. They also often tell us that the person is still very much the same person s/he has always been in so many other ways.
I don't underestimate the importance and significance of the very big part of advanced Alzheimer's patient that has been lost. What shouldn't be underestimated, though, are all the things the individual has brought to the world and to those s/he loves, and who have loved him/her. That person still has the same arms that rocked a child, the same hands that held someone else's hand, the same feet that once stood firmly on the ground, and the same beating heart that not only sustains the life of someone who has lost so much, but the life of someone who has given so much that can, and won't, ever be lost.
I don't think life (or disease) can ever make someone "not be him any more". I think they can separate him from the "him" he used to be, alter the ending to his story, and put him in a place that nobody who has never been there could ever understand. I don't think, though, that life and/or disease can ever turn someone into being anyone or anything other than himself (disabled and different from his "old self" as he may be).