Would I be able to challenge a Will?

Two weeks before my grandfather passed, myself and my siblings were removed from their will. We were then given $500 each as a good gesture. They told me that my grandparents took us out of the Will on their own 2 weeks before my grandfathers passing, however, I find this unlikely. I believe the alterations were made to the Will and my grandfather was not aware of the changes made when he signed. Will there still be record of the old Will? How hard would something of this nature be to prove? My grandparents always told me that when they passed, my siblings and I would be taken care of. I was very close with them. It is not believable that my grandfather wrote us out of the Will, two weeks before he passed without being unaware or coerced. What would I need to prove this? What would my first step be? What is the liklihood of even winning a case like this?