I recently transition from using Downcast to Overcast. Unfortunately, I was pretty far behind listening to some of my podcasts, and Overcast was not able to "see" some old episodes of some podcasts (e.g. Security Now!, since TWiT only keeps the latest 10 episodes in the feed). I create a javascript/php Manual Podcast Feed Generator that allows for the quick creation of non-updateable podcast feeds. All of the information is encoded in the URL, so I don’t need to store data.