Some people have been known to take a perfectly good attorney-prepared estate plan and tinker with it on their own. A classic example is crossing out names and writing in different names by hand. Another example is crossing out a bequest, either because the testator has changed his mind about giving it to a particular person or he no longer owns the item.
A recent court case in California highlighted the problem with this technique. In Pena v. Dey (Cal. Ct. App., August 30, 2019), the grantor created a revocable trust and amended it to name 15 different beneficiaries. He later decided to amend his trust for a second time to reduce the number of beneficiaries and modify their respective percentages under the trust. He made handwritten interlineations to the first amendment, naming a friend and others as beneficiaries. Wisely, he then mailed to his attorney the original trust, the first amendment with the handwritten interlineations, and a Post-it® note asking his attorney to formalize his notes into a second amendment for his signature. Unfortunately, Anderson died without signing the second amendment. The question arose whether the handwritten interlineations constituted a valid amendment. The court ruled that they did not because they did not satisfy the formalities required by the trust itself.
The same goes for will amendments. Don't think you can take your will out of the safe and cross out provisions and interlineate new ones and call it good. Every state requires that wills be signed with certain formalities (usually two witnesses), and that applies to amendments too.
If the changes you want to make to your existing estate planning documents are at all important to you, hire an attorney to ensure that the changes are done right and will hold up in court.