When my sister that had POA (Will made when our mother was alive and father and both were in sound mind and body) could no longer take care of my father, my eldest sister stepped in to keep him. She then took my father (diagnosed with Dementia 2016) to a lawyer and changed his Will to her name and having the power of attorney and financial POA without consulting the rest of us. There are reasons my parents did not appoint the eldest as POA in the first place. Although we are not pursuing any legal action because our father is good right now, I struggle with her going behind our backs and not letting my other siblings that live there see him without her and her husband hovering over the whole time.
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?