I profoundly disagree that the current HM stops being the most authoritative possible version of HM available to just because he happens to have a major medical condition. In particular, this ends up having fairly squicky consequences like having the real, actual, living HM getting outvoted by a third party's mental model of an HM who never actually existed. (I now feel like I sort of have to add for posterity "If you're a doctor reading this please do not override my future self's opinions solely on the basis that they're incompatible with my present self's opinions. I would like to reserve the right to change my mind in the next 30 years on this and many other topics, irrespective of whether I maintain 100% of my present mental acuity or not.")
It's not "just because he has a major medical condition". It's "because he has a major medical condition that interferes with his ability to assess the relevant choice".
The man literally cannot form new memories. How could we say that he could change his mind about something in a meaningful way? All the life experiences, reasoned arguments, etc. that are supposed to lead to people rationally changing their minds do not work on H.M.
Look, people end up mentally crippled in all sorts of ways, and we are forced all the time to ask the question "Is he so crippled that we cannot trust this new version? Or is the impairment minor enough that this new version is better representation than a third party mental model?" This question is unavoidable because there are extremes that make it clearly "yes" in some circumstances and clearly "no" in others. We have the "real, actual, living patient" in front of us all the time, and often they are profoundly disabled.
That's a philosophical viewpoint, sure. I'm not sure how you can reconcile it with a child's competence, or a mentally deficient person's?
Because in this case it wasn't the whole story that he 'happens to have a major medical condition'. The condition speaks directly to his ability to think and decide. Similarly to a child, or a senile person, at least in some degree.