Elder Law Attorney
You can change your own power of attorney as long as you still have capacity. The last power of attorney should be revoked. You can do that by revoking it directly in the new form. You can probably change your doctor without telling the first one, but you would be so much better off with getting your old medical records. Your new doctor will want to see a history and should. Again, you can probably change a lawyer without telling the first lawyer, if there is no continuing cases, but there are some ongoing matters that you would need the records that have already been worked on.
You noted grandfather's concern. Does your grandfather wish to change all these at once? If so, you might want to ask him why. It would be unusual to change all these important items in his life without a reason.
If it is your concern about changing the power of attorney, doctor and lawyer for your grandfather, unless you have guardianship over him, you can't. And the court granting guardianship would want to know why you were changing all three of these items.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Estate Planning Attorney
While you can change doctors and lawyers without notifying the prior for reasons discussed in other posts I wouldn't advise it. As for the Power of Attorney, until you tell the agent named in the first Power of Attorney that a new Power has been signed revoking the first, that agent is justified in still using the Power, they do after all still honestly believe they are the agent. To make an effective revocation of the prior power, notice of the revocation should be given to the prior agent and to any institutions who were given a copy of the prior power.
Very truly yours,
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