The Relax NG homepage lists the Sun RELAX NG Converter which apparently is able to convert an XSD file to a relaxng one - but the link is dead and a blog entry about secondary sources seems to be outdated as well.
I am not interested in a web-based converter.
Btw, the XML schema I want to convert uses inheritance.
Kohsuke Kawaguchi's Relax NG converter (aka the Sun Relax NG Converter) is currently available at http://java.net/projects/msv and http://search.maven.org/#search%7Cga%7C1%7Cg%3A%22net.java.dev.msv%22 (thanks to KK for the information).