Complete a JSON Feed Aggregator for WordPress Site

1. I have a partly completed JSON feed aggregator that creates new custom post type posts from the feed items; while the new post creation import part works fine, what I need is to have it be able to update current custom posts if there are changes in the JSON without overwriting specific advanced custom field values on the individual WordPress post if filled. 2. The scrape event is currently triggered manually and if at all possible I would like to set up a polling event to check the timestamp of the JSON feed and import automatically if it's been updated. As most of the work is already done I would prefer not to expect a complete rewrite of the code. The requested changes are just that - updates to the code - and I'm happy to explain further. Please ask questions if interested.