Assuming your state has adopted the model language of the Uniform Commercial Code, you could have paid the items in good faith even if the dates had not been changed.
§ 4-404. BANK NOT OBLIGED TO PAY CHECK MORE THAN SIX MONTHS OLD.
A bank is under no obligation to a customer having a checking account to pay a check, other than a certified check, which is presented more than six months after its date, but it may charge its customer's account for a payment made thereafter in good faith.
If your customer did not want these checks paid, they should have placed (and possibly renewed after 6 months) a stop payment on the items.
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