Possible for DPOA ability to change the Will?

Absolutely not; it's beyond the scope of your authority to change her wishes. If she didn't want the other grandchildren to inherit, she had her reasons, whatever they were and those need to be respected.

Assuming she had a good attorney when she executed the Will, he/she would have established enough of a rapport that he/she could make some reasonable estimation of her sound mind. It's therefore not for anyone to challenge or change her desires.

It is difficult being a neutral party within a family, and I can understand that this is difficult for you. You want to keep peace in the family.

On the other hand, no one has the absolute right to expect to inherit anything, and the grandchildren who were left out really have no justification to be upset.

No, it is not possible at this point. The Will can be challenged in court after her death. More often the beneficiaries of the estate will pass on a portion of their inheritance as a gift to those who were left out.

I'm sorry for your situation. This is one reason that wills need to be updated. However, a person with durable POA does not have the authority to change or to write a will. Is your mother's dementia so bad that she is no longer competent to change the will herself?