Sprint has comes up with the latest update on EVO 4G, although there is no Froyo yet. And while the original update didn't brick everyone's phone, Sprint thought it would be a good idea to pull the original, and then release this one instead. With it, the main key point is faster WiFi.
Don't worry about this sudden updates because it probably won't break your phone. Rather, help you even more. Seems like Spirint is trying their best to keep its customers happy, and making it so their phone works faster, in any capacity at all, is one way to do that. Of course, not bricking your phone is a good way to make that happen, too.