We don't support username changes, but instead we offer an “alias” option. An alias for your username is used in the URL for your blog page and RSS feed. For example, if your username is “johnsmith”, but you don't want your name to appear in the URL to your libsyn stuff, you can use an alias to hide it. So, for an example, if you want “supercool” to be your alias, then email your username and desired alias to support@libsyn.com. (This will, of course, be automated in the future).
Your blog and RSS feed will still be available at http://johnsmith.libsyn.com and http://johnsmith.libsyn.com/rss respectively, but they will also be available at http://supercool.libsyn.com and http://supercool.libsyn.com/rss. Whichever URL you choose to advertise is up to you.
To use the alias option, drop us an email at support@libsyn.com. There is a one time $5 fee and you get up to 3 aliases per account.