You can easily do this by an amendment. Each beneficiary can be treated differently. You do not need to re-do your entire trust agreement. Rather, you can make this change by doing an Amendment. In the Amendment, you can reference your original trust agreement, state that other than the changes in the Amendment all terms and provisions of the original trust remain in full force and effect, and then specify which provisions of the original trust are being changed. You want to have the Amendment signed, witnessed, and notarized in the same format as the original trust. Then, make sure you keep the Amendment with the original trust so whoever gets your trust at your death will know the Amendment exists. You can do this yourself or you can seek the guidance of a lawyer, but you are not obligated to engage a lawyer to do this for you.
In the trust, with regard to your gambling son's share, you will want to state that it is to be held in trust for him rather than being distributed. Distributions shall be made by the trustee to provide for his health, support, maintenance and education, taking into consideration her age, education and station in life, but not otherwise.
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