On two of our systems I have attempted to upgrade from Jenkins 1.642.4 to 2.7.1 LTS, and in both cases, Jenkins would not start up.

Here is the error message in the log:

INFO: Downloading jenkins.war
Jul 14, 2016 8:45:47 AM hudson.model.UpdateCenter$DownloadJob run
INFO: Installation successful: jenkins.war
Please use CMSClassUnloadingEnabled in place of CMSPermGenSweepingEnabled in the future
Running from: /usr/share/jenkins/jenkins.war
Exception in thread "main" java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at Main._main(Main.java:246)
at Main.main(Main.java:91)
Caused by: java.lang.IllegalArgumentException: Unrecognized option: --preferredClassLoader=java.net.URLClassLoader
at winstone.cmdline.CmdLineParser.parse(CmdLineParser.java:53)
at winstone.Launcher.getArgsFromCommandLine(Launcher.java:359)
at winstone.Launcher.main(Launcher.java:332)
... 6 more