Locking the barn door before the horse is stolen

What is the name of your state (only U.S. law)? Indiana

I have a will, a living trust. and, I think its called, a contingent power of attorney in case of incapacity.

If, at some point, I should be declared legally incompetent, can the person designated with my PoA, change the terms of my Will and or Trust? If that is possible, what steps can I take now to forestall that ever happening?

I am concerned that my well reasoned and thought out estate plan might be subverted as a get older and start to decline. I understand there is no perfect solution. I am interested in practical ideas.

Thanks for any and all ideas.

I can't speak to Indiana specifically, but, generally, an agent would not have the power to revise a will or trust unless specifically granted that power.

You could always revoke the existing POA document and write an new one explicitly denying that power to the agent.