Numenera Discovery not a new version but andan update to some of the core rules. This does mean that all content is supposesupposed to be backward compatible except for those small changes that have been made (list in the previous postthis 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 thatthen you are going to want to buy the new core book, but you don't have too.