The antonyms for the phrase "become valid" can vary depending on the context of the phrase. Some common antonyms include, "expire," "invalidate," "nullify," "disapprove," "cancel," and "annul." Another possible antonym is "cease," which indicates the ending of something. On the other hand, "become invalid" implies that something was once valid and is no longer so. Therefore, an antonym could also be "remain invalid." In general, antonyms are useful for understanding the full range of possible meanings of a given phrase or word. By considering their opposites, we gain a deeper understanding of how language functions and the nuances of meaning that different words can convey.