Any time there is a major change in your life—marriage, divorce, birth of a child—it is a good idea to update your will. Or there may come a time when you want to change your beneficiaries, or add or subtract property to be devised.
For minor changes such as changes to specific bequests or a name change or change in personal representative, you would probably just want to make a codicil. With a codicil, you can make specifics changes to certain items while leaving the rest of the will intact.
To have legal effect, however, a codicil has to adhere to the same formalities as a will: testator has to be mentally competent and has to sign the document, and there have to be two disinterested witnesses. Handwritten updates on an attested will will not suffice, and may actually cause the updates to be ignored or the will to be void.
Can a testator make a series of codicils with changes over the years? Yes, but that can be confusing for your representative who will have to piece it all together. There can also be confusion as to what is a codicil and what is a will. If a second document does not expressly revoke the previous will in its entirety or revoke it in its entirety, more than likely it will be seen as a codicil, and the rest of the will can stand as valid.
So, if a testator has made a series of codicils, or has major changes to make to the will (a new spouse or children, cutting out a beneficiary, changing family distributions, adding charitable contributions, etc.) it may be best just to draft a whole new will for peace of mind. Contact your estate attorney for help in this area.