Please correct me if I'm wrong, but it seems the log4j2.xml must be in the classpath of the program, and everything in the classpath gets packaged up into a .jar file when you make a standalone runnable .jar file of the program.
If that's the case, then the XML file cannot be changed after the program has been exported to the .jar file. Thus, it doesn't seem that there is any way to change the logging level without re-exporting the program.
Please tell me I'm wrong about this and that there is a way that I can change the logging level while the program is running by using, say, a drop-down list in JFrame so that the user can select the logging level.