Nebraska election law does not permit qualified parties to substitute new nominees past September 1. However, on September 17, a state trial court refused to prevent the Republican Party from adding a new Lieutenant Governor running mate to the November ballot. See this story. The original nominee resigned as Lieutenant Governor on September 9 and did not desire to run for a new term. The new nominee was named on September 15. The case is State ex rel Elworth v Gale, Lancaster Co., C114-3204. Here is the two-page court order.