Can we challenge a change in POA?

The essential question is whether your father was compelled to execute the new Durable Power of Attorney under duress. If your father has a lawyer, I would suggest that you See also:
When someone is incapacitated, how can another person get POA for them? inform the lawyer as to what happened. The lawyer can inquire into the situation with your father if your father is lucid. If it is determined that your father executed the Durable Power of Attorney under duress, the easiest course of action would be to revoke the Durable Power of Attorney by which your father's significant other was appointed. It would then be important to notify any financial institutions with which your father does business that the DPOA was revoked. The next step would be to determine if the DPOA appointing your father's significant other was utilitzed. If so, steps can be taken to remedy any actions taken. It should also be noted that merely because your father's significant other was appointed as an agent through a DPOA, this does not revoke your brother's appointment unless that is specifically done. Certainly contacting your father's lawyer is the best place to start to fix this situation.

Having dealt with these issues with caregiver and the "significant other" problem, Dad is the lawyers client and if invokes client/attorney privilege there goes that avenue. It was unclear if the previous DPOA was left on as a successor/alternative, but you cant have both DPOA's existing as who does anyone recognize as the real authority.

I thinnk this answer should have been directed to a certified alderlaw attorney in the state that writer came from