Eating animal is easy, mostly proteins and fat that are also in plants. The main difference I can think of now is that animals have glycogen instead of starch. Glycogen is more branchy than starch, but IIRC it's not so difficult to digest and there is not to much of it anyway.

The animals adapted to eat plants probably have a gut that is too long, so it will be less efficient, and I guess they have to remember to eat their vegetables to avoid problems.

The reverse is harder. An animal adapted to eat mostly animals have a short gut and will not extract all the nutrients from the plants. Also, plants have cellulose that is super hard to digest, so some animals have special parts of the digestive tube to let the food wait and be partially digested by friendly bacteria. (Yes it's gross, unless you think it's almost like free beer.)

Would they spend the energy if there wasn't any gain to it?

I don't feel like the article is lacking for that. It's evolution at work. There could be monkeys eating animals that can properly digest them due to some mutation or unique gut biomes. Actually eating the animals is an important first step. Monkeys that usually eat fruits are hunting and eating animals when food is scarce, that's an article worthy observation.