Do you have your own wordpress blog to power your podcast once you make the move? (highly recommended). If that's how you are planning on doing it, you would setup powerpress on your wordpress blog, and then make the move in hosting ( you will want to have a month or 2 overlap in service between libsyn and blubrry hosting) .. then move your episodes. You will have to start out with a higher limit account to get your back catalog of shows moved over. After they are all uploaded to blubrry, you can downgrade your blubrry account to what you need per 30 day period.
Once you have made the move and your wordpress / powerpress feed is good to go, contact libsyn and ask them to put a 301 redirect for your feed at libsyn to the one you have on your own blog. They have been good about doing that in the past but I can not speak for how they do things currently.
Then in your powepress settings you will want to make sure you have filled out the itunes tab with your itunes ID. I tunes will see the 301 redirect at libsyn and then switch over to your wordpress feed and your subscribers on itunes will not miss a beat. You will also want to mention in your show that your feed is changing so that the listeners that do not use itunes will know to change over from the libsyn feed at a certain date.
Whenever you make changes like this, it's best, if you can afford it, to leave both services in place until you are completely established on the new service. That way you will loose as few subscribers as possible.
-Mike