A Will that was valid when made remains valid, even if the person making the Will later becomes incompetent, or the witnesses should die. However, after the persons death, the heirs who are seeking to have the Will admitted to probate have to produce some proof that the Will was properly signed and thats where the witnesses come in. If the witnesses are unavailable, it can lead to delays and added expense for the heirs and the executor.
To anticipate the potential problems, self-proving affidavits are often used. They are statements signed by each of the witnesses under penalty of perjury, generally right after the Will is executed and witnessed, that the person making the Will did, in fact, sign it, appeared to be of sound mind, and was signing the Will freely. (In some states such statements must be signed before a notary public.) Your witnesses wont have to testify as to your wills validity when you die, for the affidavit speaks to the wills validity and serves as extra insurance that they witnessed your signature. This Self-Proving Affidavit also avoids problems that may arise if the witnesses cannot be located.
In the absence of both witnesses and self-proving affidavits, particularly if someone may want to contest the Will, it may be necessary to demonstrate to the judge that the signatures of the person who made the Will, and those of the witnesses, are genuine. That can increase legal expenses, require handwriting experts, and added to a familys grief after a death.
Among the options open for someone whose witnesses have died: (1) have the person draft a new Willeven one with the exact same provisions with new witnesses and a self-proving affidavit, or (2) have him prepare a Codicil (essentially a minor amendment) to his existing Will that states that the existing Will is still valid.