What documents should we gather if a dispute develops over estate planning when a parent may be losing capacity?

As part of the legal review, legal capacity is decision specific and can fluctuate, so a diagnosis alone does not answer whether a person can sign a particular document. For the issue described here, an existing agent cannot create a new power of attorney for the principal and usually has limited authority to alter estate planning. State law can materially change probate, trust, ownership, and family outcomes, so local counsel should verify the governing instruments and current court or title status before anyone acts in a way that may be hard to reverse.