Thanks for your enquiry.
I'm afraid the powers of a person with an LPA do not extend to changing the Donor's Will.
The only circumstances in which a third party can get another person's Will prepared is if the person has lost mental capacity and the third party can apply to Court for what is called a "statutory Will" which in effect will only make a very straight forward Will leaving the Estate to that person's next of kin.
I am sorry this may not be the answer you were looking for, but sets out the legal position.
Please let me know if you require any further clarification.