Criminal Law Questions? Ask a Criminal Lawyer.
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Changing someone's check without permission would be considered forgery in Oklahoma. Did someone witness your grandfather say you could change the amount from $100 to $400?
There is a defense to forgery that you acted in good faith and were authorized to alter the check. That is why I asked if you had a witness.
This level of forgery is considered a felony and if convicted a person can get a sentence of up to 7 years in prison. Have you considered speaking with your grandfather and giving him back the money? If he will not discuss this with you, you should consult a lawyer. This is a serious charge.
Please let me me if you have any other questions. If you do not and your are satisfied with my answer, I ask that you rate my service. Good luck.
I had one more thought. If your kid's grandfather does call the police. remember your Miranda Rights. You have the right to remain silent and not make a statement.