I think there is no such option to download or save all of your write ups on medium. If it's there than good. If not then nothing can be done. We've to save our articles in doc's or word, If at all medium vanishes suddenly & we'd be on the safer side if.
I save my articles on docs and ms-word and sometimes even email them to myself. But nothing to worry Deborah. You can republish the same content on other platforms like Quora, linkedin or Twitter.