By default just doing uninstall/reinstall of the same extension id/version will always reinstall the same jar because it's kept in local repository but it's possible to clean the local repository using something like http://extensions.xwiki.org/xwiki/bin/view/Extension/Extension+Tweak. In this case the process is
- clean using extension tweak
This works well with Tomcat 7 (you get the new behavior) but looks like in Jetty there is some kind of optimization which keep the old class somewhere...
The workaround is to restart.