Can Power of Attorney be changed?

As long as your grandmother still has enough mental competency to make a new Power of Attorney, then she is free to do so at any time. And, if there's some guy that she doesn't remember who claims that he's her Power of Attorney agent, then she may well need to make a new one as soon as possible, and to let everyone she may have contact with know that they should not honor the document that he is presenting. At that point, if she makes a new one in favor of your dad, the other person will have to try to prove that she validly appointed him and that she does not have the capacity to make a new one. If she does not currently have enough competency to make a new Power of Attorney, then your father may need to seek to have himself appointed as her conservator and guardian (in Georgia, conservator has power over someone's financial and economic affairs while guardian has power over health care issues) and to have that existing Power of Attorney revoked. Your grandmother (or your father, if needed) should consult a good estate planning attorney as soon as possible.