Bdtyktl

In my fantasy world, I have created a sentient species descended from an arboreal felid (it evolved to eat squrriels and small monkeys). They are my elves. The one problem is that I want them to be herbivores. So I would like someone to help me figure out why a carnivore descendant would do this. This world has natural magic, which sort of radiates from magical sources such as mana springs and arcanacrystals) so feel free to use that in your answer.

You could start with the panda pattern. Giant pandas are carnivores which have adapted an all vegetarian diet. Your elves are also pandalike fallen carnivores.

https://en.wikipedia.org/wiki/Giant_panda

Despite its taxonomic classification as a carnivoran, the giant
panda's diet is primarily herbivorous, consisting almost exclusively
of bamboo. However, the giant panda still has the digestive system of
a carnivore, as well as carnivore-specific genes, and thus derives
little energy and little protein from consumption of bamboo. Its
ability to digest cellulose is ascribed to the microbes in its gut.
Pandas are born with sterile intestines and require bacteria obtained
from their mother's feces to digest vegetation. The giant panda is a
"highly specialized" animal with "unique adaptations", and has lived
in bamboo forests for millions of years. The average giant panda eats
as much as 9 to 14 kg (20 to 30 lb) of bamboo shoots a day to
compensate for the limited energy content of its diet. Ingestion of
such a large quantity of material is possible because of the rapid
passage of large amounts of indigestible plant material through the
short, straight digestive tract. It is also noted, however, that such
rapid passage of digesta limits the potential of microbial digestion
in the gastrointestinal tract, limiting alternative forms of
digestion. Given this voluminous diet, the giant panda defecates up to
40 times a day.[56] The limited energy input imposed on it by its diet
has affected the panda's behavior. The giant panda tends to limit its
social interactions and avoids steeply sloping terrain to limit its
energy expenditures.

Your elves would eat all the time and be large, sluggish, antisocial and productive of large quantities of splintery feces. That is pretty much elves, I think.

Pandas just stumbled into the bamboo forest and started eating. The bamboo probably does not get much from the deal. I think if your elves are intelligent it offers another more conventionally elvish property. These elves are gardeners in the manner of the acacia tree ants.

https://news.nationalgeographic.com/news/2013/11/131106-ants-tree-acacia-food-mutualism/

Trees Trap Ants Into Sweet Servitude
A sip of nectar enslaves ants to the trees they guard.

The elves grow and protect the trees, and in return the trees give them leaves and sugar. Like the ants, the elves are totally addicted to the leaves and sugar from this particular tree and so must carry supplies with if they travel. Fortunately the sugar and leaves ferment very well, providing concentrated calories and thus even though the elves are large, sluggish and antisocial the elf liquor helps them overcome this with wild dancing and parties.

The drink is also addictive to nonelves. If humans partake, they might find themselves dancing at an elf party until the drink runs out. That takes a long time.

The change from carnivorous to herbivorous is not likely to happen quickly. For example, it would be unlikely for intelligent carnivores to just choose to go vegetarian as they wouldn't have an even approximately appropriate digestive system. You would end up with a bunch of sickly, lethargic and grumpy Elves.

What could happen would be a gradual evolutionary change from carnivore to herbivore - more likely if they were already partially omnivorous. This could be due to environmental pressures. For example, repeated boom/bust in squirrel population means that only the more omnivorous felids survive periods of starvation. Then either some environmental disaster totally devastates the squirrel (and monkey) population so felids go totally vegetarian, or now social/religious/ethical drives could push them in that direction.

As a real-world example, consider bears - polar bears are almost totally carnivore; pandas are totally herbivore; but they both descend from the same ancestral proto-bear.

But you are probably needing many thousands of years (possibly up to millions) for this to happen.

Well it's simple. Just because they only have pointy teeth does not mean they can't eat plants.

Ask any pet owner. Some cats, and all dogs, will eat mostly anything that we can eat. Yet, officially, they are carnivores.

In the wild, a decrease in the amount of available prey coupled with an abundance of plant food translates into evolutionary pressure towards herbivorism. This has happened more times than we can count on Earth. Of special notice are birds: parrots, humming birds and toucans all share a common ancestor with all other birds, which is believed to have been a carnivore theropod.

We actually descend from similar species, as primates come from the Eutheria
https://en.wikipedia.org/wiki/Eutheria small carnivores/insectivores.
It's been said that our brain developpment as specie was thanks to hominids eating marrow and that that makes us omnivores but you can find sources of fat in plants too, convenient for other types of evolutionary (and magicaly evolutionary) adaptations.

Farming and direct choice

It is a lot easier to farm plants than it is to farm animals, and easier to store them as well. You don't have to feed the dried apples you have in your storehouse over the winter. Cattle still have to be fed, which means you're growing and storing the food anyway.

Instead of evolving this way, your elves deliberately chose this path. While they may have been carnivores or partial omnivores, they steered their culture towards farming - And while obligate carnivores require meat, your elves were able to harness the nature magic to change their whole physiology to accept and thrive on plants - And to have those plants produce more and better nutrients. This is why elvish travel bread is so good for its size.