That's a simple fix using the "Find & Replace" feature. Go to your listings tab and click on Saved Listings. Check the box next to the listings you want to change and then hit the "Find & Replace" button on the top of the window. That will take you to a new page where it asks you what field you want to change. You will first need to change your listings from Auction to Fixed Price. When you are done with that, you can then go through the same process from the Saved Listings page and change the listings from 7-day (or whatever the previous timeframe was) to the new 30-day format.
Hope that helps a bit...good luck!