Generally, there are two ways that a codicil (an amendment to a will) may be valid under Texas law.
The first is a formal codicil executed by the testator before two witnesses and signed by the witnesses. A formal codicil may or may not have a self-proving affidavit which is notarized. If it does not have the self-proving affidavit attached, then the witnesses will have to testify before the probate court that they witnessed the execution.
The other type of valid codicil is called "holographic.". That is, the material terms of the codicil are in the handwriting of the testator and it is signed by the testator. Typically, at least one disinterested person will have to testify to the probate court to identify the testator's handwriting.
In most instances, an executor must be represented by legal counsel in Texas. As such, you should consult with a probate attorney in the county where the testator died to determine the validity of the codicil.
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I will add just a short comment to the excellent answer above. In order to amend the terms of the Will, the Codicil must meet all of the requirements of a Will. For example, it must express "testamentary intent". That means that it must clearly state that it is meant to pass property upon the death of the person signing the Will.
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