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.)
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.)