You don't need the "already" because it's implied by the fact that the announcement hasn't happened yet. I (speaking American English) might say something like, "Don't be too excited before it's announced" or "don't get too excited before you know for sure!" There's an expression we use, at least in the U.S., for this sort of thing: "Don't count your chickens before they hatch!" It comes from a fable about a young girl carrying a basket of eggs to sell at the market. As she walks, she dreams of what she will do with the money she will get from selling the eggs. Because she is daydreaming, she trips and drops the basket. All the eggs break. The money and her dreams are gone. So "don't count your chickens before they hatch" means "don't feel too confident about something before it actually happens."