My father who has early alzheimers and dementia had his POA and health care POA changed over the phone while staying with a sibling. Is this legal?

You likely will need an elder attorney to look into this. It certainly sounds fishy. Be prepared for a legal fight if the person with the POA wants to squabble. You'll need the cooperation of the attorney they called to make the change, as well, I'd assume.

Carol

We were just told that the POA we have for my parents needed to be recorded with the County we live in. Maybe if you had the original POA recorded with your County Records department and then take it to an attorney that deals with elderly issues it might help.

What does the sister who is caring for him now say about all this?

The easiest way to get "back control" of his finances and health care issues is to start to care for him yourself and help with managing his finances for his daily care, including contributing your own cash to support him. Please don't use Alzheimer's as an excuse to gain control without the responsibility.

If he is in the care of your sister, what's to preclude him from re-changing the POA? If he was okay a few months ago, he's probably okay now.

If he wasn't okay a few months ago, it's easy to argue he's not okay now.
If his capacity is going to be contested by anyone - it can be - it will drain whatever assets he has, think lawyers and un-ending billable hours, capacity examinations by neurologists, court-appointed physician to make an evaluation, civil court case, depositions, discovery (gathering of documents), arbitration meetings, etc.

Alternatively, your sister caring for him now could apply to be his conservator (legal guardian for adults).

Filing for conservatorship trumps POA and the courts will support a conservator to with going after the sister to recoup money taken or surcharge her for the amount she took. However, more and more, courts are removing family members who are conservators and placing professional fiduciaries in charge, which can turn out very badly, which is what happened to me. Now my mother is being neglected, alienated from family members, and the professional fiduciary and his counsel have obtained over $150,000 from the family trust that I am beneficiary of and he sold our family home for 1/2 its value, took, gave, and threw, away all family tangible assets, photos, heirlooms, and historical documents, and surcharged me $50,000 for paying dad's funeral bills from the trust for the few months I was trustee. Please see an attorney specializing in estate litigation and research everything the attorney suggests because conservatorships are easily abused by professional fiduciaries and attorneys alike, as there are no governance in place for them. Also, if you become your mom's conservator, make sure you thoroughly research the probate codes in mom's jurisdiction and know what you are expected to do as her conservator or they will remove you and replace with a stranger professional fiduciary and his counsel who only care about how much money they can obtain and not about the conservatee. I am sorry for the pain you feel over your sisters betrayal and I know how it feels to not be able to protect a loved one. Good luck to you.