You need to rebuild classes.jar. The simplest way to do this is to get the latest source out of SVN and use the ant build file. This is particularly easy if you use Eclipse.
You can rebuild classes.jar, by changing directory to lejos_nxj\src\java\classes, recompiling all the java files and using jar to rebuild classes.jar, but you will need to work out the parameters to javac and jar. You will then need to replace classes.jar in lejos_nxj\lib.