This page discusses how you upgrade from Hudson to Jenkins.
Please make a backup, especially the Hudson home directory, that contains the most important information, such as jobs, configuration and build history. In the unlikely event that something goes wrong you can recover from the backup.
Jenkins is basically a drop-in replacement to Hudson. It's the continuation of the same code base, in same package structure. There has been no major surgery since the rename, and the rename really only affected what's shown in the UI. As such, it understands the same set of environment variables, same system properties, and the same information in the home directory. So if you rename jenkins.war as hudson.war, and simply overwrite your hudson.war, the upgrade is complete.
Current users of the Hudson debian package should follow the instruction here for the new users. When you do "apt-get install jenkins", it will uninstall the hudson package, transfer your /var/lib/hudson to /var/lib/jenkins.
Current users of the Hudson rpm package should follow the instruction here (Red Hat) or here (openSUSE) for the new users. When you do "yum install jenkins" or "zypper install jenkins", it will uninstall the hudson package, transfer your /var/lib/hudson to /var/lib/jenkins.
See the caution about the Debian package above, with regard to what happens to /var/lib/hudson and the hudson user. In RPM packages, the configuration file is in /etc/sysconfig/... instead of /etc/default/....
If you've installed Hudson as a Windows service, stop the service and overwrite hudson.war with jenkins.war, and restart the service. No additional steps are necessary.
If you prefer, you can uninstall Hudson as a service (by executing hudson.exe uninstall which produces no output on successful execution), and re-install Jenkins as a service from GUI. During this process you'll be asked to specify the installation directory. Choose the same directory you've installed Hudson so that your data gets carried over.
If you've installed hudson.war into a servlet container that you administer, download jenkins.war and deploy it as hudson.war.
If you prefer, you can just undeploy hudson.war and deploy jenkins.war. Jenkins recognizes HUDSON_HOME environment variable and system property, so it'll automatically find your existing data.
You can install Jenkins update center in your Hudson so that you get update notifications of future Jenkins releases. You can use the built-in upgrade mechanism in Hudson to smoothly migrate to Jenkins. To do so, go to "Manage Hudson" > "Plugin Management" > "Advanced" > "Update Site" and specify "http://updates.jenkins-ci.org/update-center.json"
While you are at it, please check this document and retrieve/create your account on jenkins-ci.org, so that you can file issues, participate in Wiki, and so on.
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