Legal Question in Wills and Trusts in Maryland

Your question isn't clear. There is no formal "bill of rights" for estate beneficiaries although after the Testator dies beneficiaries may have certain rights like notice afforded by law. You can view the Estates & Trusts article of the Maryland Annotated Code for more details.

A competent individual who makes a power of attorney can change it at any time they want so long as they are still competent. Once the principal becomes incapacitated they no longer have the ability to change this and others cannot "remove" the Agent unless there is a clear breach of fiduciary duties. It is rather difficult, expensive and time consuming to undertake such litigation.

Note that an estate/will doesn't come into play until someone dies while power of attorney is only good while the principal is living and stops once they die.