I am one of 5 siblings and living in another state. Both my sisters taking care of my father and did the best they could, but I'm struggling with POA change. Any advice?

In my opinion it only makes sense that the person who has to do the care also has the legal authority to make the decisions. As Countrymouse has said, being diagnosed with dementia is not the same thing as being incompetent, so as long as the lawyer was satisfied your father understood what he was doing at the time he was perfectly capable of changing his POA and amending his will. I gather your relationship with oldest sister is not the best, but she did step up and took dad in when the other sister could no longer care for him. As for not being able to visit freely, you can hardly expect her to vacate her home whenever you choose to visit. And I might point out that my mom is in a nursing home and I have absolutely no privacy to visit there either because mom has room mates and the lounges are always occupied... unfortunately it is often just another of those things we have to accept as our parents age.

Gisela, important point number 1 is that a will and a Power of Attorney are completely separate documents doing completely different things.

You say your father was diagnosed with dementia in 2016; but has he also been declared incompetent? Dementia isn't an on/off switch. In the earlier stages, a person with dementia can still be capable of weighing up and making a decision. So if the powers of attorney were altered, this can only have been legitimately done with your father's full knowledge and valid consent. Is that what you want to challenge?