Will Google read Schema.org markup in the form of JSON-LD to automatically update a product feed?
It says that it uses Microdata but it doesn’t say whether or not it will look at JSON-LD when updating the feed.
For using Automatic Item Update you must set the microdata from schema.org in your HTML, but I am afraid you won't be able to use JSON-LD.
Also, keep in mind that with the Automatic Item Update you won't need to update your feed anymore, it is just for cases where the price/availability of the product in the landing page has changed but you haven't updated yet your feed, but you still need to provide feed with accurate information.