If you are using feedburner, you will be able to keep using feedburner. Install powerpress, configure everything and then go to feedburner and change the feed that feedburner takes to the new powerpress feed for your podcast and you shouldn't loose anything. You may have to go back to your old episodes and make sure that powerpress picked them up.
The nice thing about feedburner is if you change feed addresses at your website, you can just change it at feedburner and nobody will notice
Let us know if you need more help with this.
-Mike
mike [at] rawvoice.com