Creating a will feels like a final move. It seems like something that, once written, can't be changed or challenged. And yet, both of those notions are untrue. After creating a will, the testator may decide to alter things about his or her will, may that change of heart occur months after the will is created or years later.
Making changes and altering heirs or beneficiaries can cause a lot of strife and chaos when a will is executed, and it may even lead to someone who is named in the will (or used to be named in an old will) to contest the provisions and rules set forth in the will.
It is important to realize that not everyone can challenge a will. It has to be someone named in the will, or someone who has an interest in the will. These parties could then mount a legal challenge against the will -- though it is important to realize that most wills go through without an issue.
Should someone decide to challenge, there are a number of grounds upon which they can form that challenge. They could argue that the testator did not have the testamentary capacity to form his or her will, or that there was undue influence to change the will to benefit some while leaving others out. The will could also be challenged if it was fraudulently made, or if another will exists, thus invalidating the first will. The provisions of the will must also be legal and appropriate in the state where the will is valid.
Source: FindLaw, "Reasons to Challenge a Will," Accessed Sept. 30, 2015