Federal regulators have upgraded a product recall of Lay’s Classic Potato Chips to the highest risk level for a batch that may contain undeclared milk ingredients. Last month, Frito-Lay issued a recall warning that more than 6,300 of its 13-ounce bags may contain undeclared milk posing a risk of a serious or life-threatening allergic reaction. On Monday, the U.S. Food and Drug Administration classified the recall as “Class I,” the most severe risk level. According to the FDA, “Class I” level recalls are used when there is “reasonable probability that the use of, or exposure to, a violative product…..