n order to move iTunes to a new feed you have to have access to the old feed to be able to add a tag to it. You need to add the following tag to the top section of the feed, between the 'channel' tag and the first 'item' tag.
<itunes:new-feed-url>NEW URL FOR YOUR FEED</itunes:new-feed-url>
iTunes will normally pick this up quite quickly but you should leave it there for subscribers for at least 2 weeks - when they next check their iTunes application will transfer its attention to the new feed, and indeed this is the only way of retaining your subscribers. Be very careful when typing in the new URL because if you make an error and redirect iTunes to a non-existent URL you will not be able to retrieve the situation and your podcast will cease to function.
Since it's necessary to have access to the old feed in order to be able to move the iTunes Store to a new feed, problems arise if you don't have access to the old feed - either because it's on a server which has become defunct or you can't access for any reason: or because you are using one of the online service which write the feed for you and doesn't allow you to modify it directly. In this case you are rather stuck, and have only two options. The first is to abandon the old feed altogether. You should get the old podcast removed from the Store - instructions at http://www.apple.com/itunes/podcasts/specs.html#changing - and submit your new feed as an entirely new podcast. You will probably need to make a small change in the title, as a bug in the Store software may tell you that the podcast has already been submitted: and of course you will lose any subscribers or comments.
The other option is somewhat uncertain. iTunes Support is notoriously unhelpful, often sending back form answers which often don't deal with the actual problem, or referring the questioner to the Discussions Forum (which may have sent the questioner there in the first place): however it has been known for someone to persuade Support to shift their podcast to a new URL, so it's worth trying - but don't get your hopes up. To contact iTunes Support, go to http://www.apple.com/support/itunes/ - click on 'Other iTunes Store Features' in the list and then on 'Podcasts'.
To check what iTunes is up to, click 'Subscribe Free' in the Store page for your podcast. Go to the Podcasts section in the sidebar: you should see your podcast listed in the main pane. Control-click (Mac) or right-click (Windows) the line for the entire podcast (not an episode) and choose 'Copy Podcast URL'. When you paste into a text program you will see which feed iTunes is working from.
