Numenera Discovery not a new version but an update to some of the core rules. This does mean that all content is supposed to be backward compatible except for those small changes that have been made (list in this other answer).
My group and I have had no problem using the new core rule book with an adventure which was started with characters created from the original book.
If you're a big Numenera fan then you are going to want to buy the new core book, but you don't have too.
