The important thing is to keep the old feed working while the new feed works already for 1-3 weeks.
If feeds work you can set the
<itunes:new-feed-url> tag in the old feed and point the iTunes server to the new URL. This should result in a seamless URL change. (see also http://www.apple.com/itunes/podcasts/specs.html#changing)
I don't know podcastgen. But it is possible that this software contains a setting for the
There is a further issue besides changing the feed URL.
podPress has no import function. That is why you will probably need to add the episodes manually to the posts again after you have imported the post from the podcastgen blog.