Is replacing old POA name on docs (with notarized sig) all that's required to legally change POA for my parent?

As others have indicated, no, you cannot do the change it that way. Your profile says mom has dementia. IF and ONLY IF she is in the earlier stages, and is still competent enough to understand can she sign new documents to assign another POA and revoke the current one.  This would still require assistance from an attorney.

If mom is not competent to understand and sign, IF there is a secondary POA already assigned, there should be a way for that person to assume the responsibilities (but you would likely need LEGAL advice as to how this works and how you can revoke the other person) and then present the updates to any/all who had the original POA assigned (in particular the revoke of the original POA, because the document they have would list the alternate - you just need the proper paperwork showing the first POA is no longer applicable - legal advice likely is necessary!)

If mom is not competent and there is NO alternative POA already assigned, you will have to petition the court to gain guardianship and stewardship (first is for the person, second is for the financials.) This can be a lengthy and expensive process, will require an attorney (recommend Elder Care) AND requires regular reporting to the courts. You would then have to present the court documents to any/all who had the original POA information.

I think your best bet would be to consult an Elder Care Attorney, for any of the cases above! They will know what your best (or only) options are.

I had to fire my POA. I sent a registered letter so receipt had to be signed. That ended our relationship (thank god). Then I went to every single institution, bank, credit card people, everyone, and notified them that she was "dead" and then provided then with the name of the new POA. In many cases I had to provide a certified copy of my new POA form showing the new person. I had no problem. This took care of everything.

I've never heard of someone revoking another person's POA for someone. From what i understand only the person who wrote up the POA can change it to another person having the POA for them and thereby revoking the previous POA who must be informed of that action.

I went to the library and go in a form,which I could use to revoke Power of Attorney,then had it notarized at my bank.It cost me a total of 10 cents.All that was asked (mind you I had power of attorney revoked from my ex sister in law over my 24 yesr okd handicapped son) If they understood what this meant and if they were okay with it.

It is not that simple. Your mother will need a new Durable POA and Medical POA with the name of the new person and a secondary person which is signed and notarized. Then a letter is sent to the former POA informing her that she is no longer the POA. And, yes you will need to inform all financial institutions and investments of this change. A change in the Medical POA will need to be shared with her doctors.

Also, Medical POA and Durable POA are not the same as guardianship. Does someone have guardianship over your mother? If so the POAs are null and void.

If guardianship exists, you can't just sign and notarize to make a change to guardian. You have to go back to court and have a judge appoint new guardian. If your parent is not competent, they can't sign a new power of attorney. You cannot just replace names in existing documents and have it notarized. Please consult an attorney as to the exact procedure your state requires so that you don't cause problems. Durable power of attorney etc. cannot just be changed by a signature. If there was a secondary power of attorney named, that person can step up. But there are legal protections in place for persons under guardianship just in case the transfer was for nefarious purposes.

A completely new POA needs to be designated. This is done by drawing up a new document (dated) which both designates the POA and states it supersedes any previous assignation.

This must be signed by Mom in front of a notary.

I would take or send copies of the new POA documents to all financial institutions just to be safe.