I doubt it was the teeth that changed. Look at the tusks of the walrus -- for digging in the mud. Look at the sharp teeth of the squirrel -- for cracking nuts and such. We have some clues today with things such as soy to show us that some forms of plant life do offer complete proteins. Coconuts are not easily broken. Stripping bark off some plants in order to get to the good stuff underneath is not a lightweight task. How much plant life was destroyed for good in the Flood?
The teeth that are used for tearing meat now could just as well been used for tearing bark and/or breaking nuts before the Flood.
We have a Sheltie mix and two German Shepherd mixes. All adore the blackberries and will suffer the scratches and scrapes of outrageous vines to get to them! Our Sheltie will eat tomatoes and grapes off the vine and loves to dig up the carrots! All of them will supplement their dog food and meat diets with grass. None of them throw it up. All of it is digested. I guarantee that. I'm the one who cleans up the back yard daily!
As far as it being necessary to chew leaves thoroughly before they can be digested, baloney. Stomach acids are wonderful things! Any stomach acid which can take on meat can most easily take on plant life!
Keep in mind that humans do not grind our food. We smash it between our teeth to break it up some, but we do not grind it. Go ahead, TRY to move your jaws side to side when you eat a salad next time!
I have no trouble at all with the idea that any and all of the large animals were at one time vegetarian.