You would have to prove that your father lacked the mental capacity to know the nature of his act or that the change was a result of undue influence. A person can be very sick and still have the capacity to change a trust or a will. You should speak with a probate attorney about the details of the transfer and see if you have a claim.
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First, you are going to need an attorney. Contesting a will is not an easy process, whether you are claiming undue influence or lack of capacity. YOu would need to file an action in the probate case and there are strict time limitations on how long you have to do this. Don't wait. Contact an attorney now.
You'll have to contact an estate planning/probate attorney with the details of your particular situation. An action will have to be brought before the probate court to demonstrate that the deathbed change in terms was a valid one. My colleague hinted at your father's competence, which is just one of a few ways this circumstance could be problematic.
In Michigan, contesting mental capacity requires a showing that, at the time that the will was executed, the person was not capable of understanding the nature and extent of their property, the natural objects of their bounty, the disposition of property that they were making, and that they were not capable of forming a coherent plan from the above. It does not take into account actual knowledge but merely capacity - therefore the bar is quite low and it is difficult to challenge.
There are other common challenges to a will, including undue influence, fraud, improper execution, and the existence of a more recent will. I strongly advise consulting with a qualified local probate attorney to review your situation in greater detail and to see if you have any valid claims.
I agree with the others. I specially agree with Mr. Sloan regarding other common challenges including but not limited to undue influence, fraud and improper execution. How long has the attorney been doing work for your dad? Who called for the attorney? Did your dad?
I would first determine whether you want to go through the cost and expense of litigating the validity of an amendment that, based on the information provided, only changes the nominated trustee. It does not appear that the distributive provisions of the trust were altered. Any trustee is going to have the same fidicuary duty to administer the trust for the benefit of the trust beneficiaries.