Here a little guide on how to update javassist in WAS. This was tested with WAS 8.0.0.1.
The javassist.jar in WAS_HOME/plugins has to be replaced by the jar from: http://search.maven.org/remotecontent?filepath=org/javassist/javassist/3.18.1-GA/javassist-3.18.1-GA.jar
Keep the old one as backup.
Invoke WAS_HOME/profiles/PROFILE_NAME/bin/osgiCfgInit.sh and then WAS_HOME/profiles/PROFILE_NAME/bin/clearClassCache.sh or the *.bat files on windows machines.

Thanks to this post I found out how to do it: http://ramirezag.wordpress.com/2014/02/27/hibernate-4-2-8-and-websphere-8-5-5-1-issues/