There are certain species that are herbivorous but carnivorans (belonging to the order Carnivora). They have evolved away from carnivory and completely towards herbivory. These include the giant panda, red or lesser panda, bushy-tailed olingo, kinkajou, and spectacled bear. Given that this change has occurred without human agency many times across diverse lineages, it likely will not be too difficult to modify, via selective breeding and genetic engineering, all carnivorous species to evolve herbivorous diets. Thankfully carnivorans generally exhibit a relatively high degree of phenotypic flexibility making these diet switches possible.
“Taxa from distantly related lineages have repeatedly developed adaptations to a feeding behaviour that represents the opposite extreme of the ancestral carnivorous condition (Van Valkenburgh, 2007)."
Van Valkenburgh, B. 2007. Déjà vu: the evolution of feeding morphologies in the Carnivora. Integr. Comp. Biol. 47:147–163.