The antonyms of "became similar" are "became dissimilar" or "became opposite." When two things become similar, they become alike, comparable, or have common features. However, when they become dissimilar, they become dissimilar, diverse, or have contrasting features. For instance, two pieces of artwork can become similar if they have similar features or styles, and they can become dissimilar if they have different styles or themes. Similarly, two people can become similar if they have similar personalities or interests, and they can become dissimilar if they have different traits or preferences. In conclusion, the antonyms of "became similar" are "became dissimilar" or "became opposite.