Once the durable power of attorney is in place, can anyone override the DPOA without authority?

ps, does it have to be a court of law? Would a letter from two different doctors stating that the elder is incapable of making financial or medical decisions suffice? Interested for future reference.

All it takes is convincing the elder to sign a piece of paper appointing another DPOA and revoking all previous ones. Unless the elder has been determined incompetent in a court of law, their signature is valid.