When to inform family about dementia diagnosis? Rationale is to explain away loved-ones behavior changes.

The only one making the decision to divulge this info is the person themselves or whoever has PoA for this person and is caring for them. I think the diagnosis is on a "need to know" basis, for whoever comes in contact with him very regularly, as his unexplained different/changed behavior would maybe drive dear friends/neighbors away.

If financial predators are a concern, the LO or his durable PoA will need to start locking down all sensitive areas, especially anyone's ability to manipulate LO into changing his will or PoA. Having an official dementia diagnosis in the medical records will help prevent financial abusers from getting away with it for long, but they can still scam him out of money, take his personal possessions, cash, passwords, etc. Make sure his ATM and credit cards, blank checks, passport, driver's license, etc. are all locked away in a fireproof safe and only trusted people have the keys. All this needs to now be of concern to the person or PoA, no matter how "mild" the LO's dementia may be at this point. This abuse happens all the time, from within family and extending outward to friends, neighbors and complete strangers. You can read all about it on this forum.

I am not one to make carte blanche announcements. If asked I will answer truthfully.

One thing I have learned about this disease is that everyone who has it progresses differently, some faster, some slower and in every different way.

I personally would tell my nearest and dearest as soon as I got the diagnosis, or at least as soon as I was able to internalize it and felt up to speaking about it. As for extended family and casual acquaintances - I would decide with each individual whether or not they need to know at all.