Classes of words in languages tend to be either "open" (accepting new members readily) or "closed" (rejecting new members). This distinction is fairly easy to see: compare how readily English accepted the verb "to google", versus the hundreds of years of resistance to singular "they". So descriptively, verbs in English are an open class, while pronouns are closed.
But have we ever observed a class of words changing from open to closed, or from closed to open? Or an intermediate "half-open" stage, where new words are accepted but not commonly used or always seen as foreign?