Most animals actually have very similar nutritional requirements, at least with respect to what nutrients they need, if not exactly how much of each. However, obligate carnivores must eat meat because there are nutrients they're body cannot synthesize, so in the absence of the ability to manufacture these nutrients industrially, they must get them directly from eating other animals. Over many, many generations, the biological processes that produce these nutrients become broken in carnivorous species, since they are redundant. For example, cats cannot synthesize taurine from other amino acids.

It's entirely possible that your carnivorous species only lacks the ability to synthesize one particular nutrient; let's call it vitamin X. In that case, all your carnivorous species needs to do to stop eating meat is to find non-meat foods which provide all the other nutrients vitamins A through K, and which provide similar macronutrient ratios. They'll probably need to eat a lot of starchy and high fat vegetables. They might need to process them somewhat to get the macronutrient ratios correct, such as by washing the starch out of flour to leave the protein.

Then it's only a matter of manufacturing vitamin X. The best way to hand-wave this with roman era technology is going to be some sort of fermented food. For instance, vitamin B12 is synthesized in modern times using bacteria. It's not hard to imagine your race stumbled upon some fermented beverage, grain, or other food that happens to contain a bacteria, mold, or other fungus that produces vitamin X. Of course, they don't know what vitamin X even is, but they may have figured out that the symptoms associated with not eating meat appear to be alleviated by eating this fermented food.

Most animals actually have very similar nutritional requirements, at least with respect to what nutrients they need, if not exactly how much of each. However, obligate carnivores must eat meat because there are nutrients their bodies cannot synthesize, so in the absence of the ability to manufacture these nutrients industrially, they must get them directly from eating other animals. Over many, many generations, the biological processes that produce these nutrients become broken in carnivorous species, since they are redundant. For example, cats cannot synthesize taurine from other amino acids.

It's entirely possible that your carnivorous species only lacks the ability to synthesize one particular nutrient; let's call it vitamin X. In that case, all your carnivorous species needs to do to stop eating meat is to find non-meat foods which provide all the other nutrients vitamins A through K, and which provide similar macronutrient ratios. They'll probably need to eat a lot of starchy and high fat vegetables. They might need to process them somewhat to get the macronutrient ratios correct, such as by washing the starch out of flour to leave the protein.

Then it's only a matter of manufacturing vitamin X. The best way to hand-wave this with roman era technology is going to be some sort of fermented food. For instance, vitamin B12 is synthesized in modern times using bacteria. It's not hard to imagine your race stumbled upon some fermented beverage, grain, or other food that happens to contain a bacteria, mold, or other fungus that produces vitamin X. Of course, they don't know what vitamin X even is, but they may have figured out that the symptoms associated with not eating meat appear to be alleviated by eating this fermented food.