Wills are not necessarily set in stone after someone passes away. Under some circumstances, probate courts are able to change a deceased person's will. This is called "reforming the will."
The use of the word "reform" gives a clue as to the purpose and limited scope of the changes a court can make. If the provisions of the will as written make it impossible to fulfill the clear intent of the deceased, then the court can change those provisions to fulfill the intent.
Reformation is rarely used, because it is difficult to divine the intent of someone who has passed away.
Some states do allow it for tax purposes, as Wealth Management discussed in "Reforming a Will for Tax Savings."
The idea is this: if the intent is clear that the deceased wished to minimize the tax burden on the estate and the tax laws have changed since the will was drafted, then a will can be reformed to fulfill that intent.
However, people should not rely on the possibility that a court will use reformation for this purpose.
Instead when tax laws change, people should go back to their estate planning attorneys and update their wills to reflect the new laws. That ensures that their wills do fulfill their intent, which is better than relying on a court to decide whether the wills do or do not.
Changes to federal tax laws are expected soon.
Therefore, people should begin planning to update their wills in the near future, if the changes to the tax laws will have an impact on their estates.
Reference: Wealth Management (May 19, 2017) "Reforming a Will for Tax Savings."