I don't know of any studies that have seen this sort of shift, though omnivory in Carnivora is quite common. Typically, omnivorous animals will alter their diets depending on availability of different types of items, as you've said (i.e., coyotes in wine country eating a lot of grapes), but I don't think veg loss has been dramatic enough for that to show a strong, lasting/scalable signature of any kind yet. Remains to be seen what will happen as animal ranges continue to shift.