Sure, they change their theories about dinosaurs because of new discoveries. It would be silly to ignore new discoveries because they could tell us things we didn't know before, like there being grass by 80 million years ago. They found that from looking at dinosaur dung. Because the teeth of the big plant eating dinosaurs don't seem to be good for eating grass, they think that it may not have been the main food. They mentioned an early mammal they found with teeth that could handle grass.
The last dinosaurs we've found were from 65 million years ago, so there was grass during the last 15 million years of the dinosaurs' time. The earliest ones we have found are from about 230 million years ago. So the dinosaurs lasted for about 165 million years. If there was grass for the last 15 million years of the dinosaurs' existence, then there was grass for only about nine percent of the dinosaurs' existence.
The plants that were around before dinosaurs included ferns, conifers, ginkgo trees, which has fossils from 270 million years ago, horsetails, which had reached heights of 30m tall, and the now extinct seed ferns, which disappeared by the end of the Triassic. Flowering plants developed during the Jurassic and diversified during the Cretaceous. So there was plenty of food for plant eating dinosaurs before grasses developed.
Dinosaurs
Plants