How to migrate from Log4j 1.x to 2.x

Issue

• I have application that is being written using log4j2, to take advantage of some of its new features. One of the
  libraries it uses is a bit older and was built with log4j 1.x. I'm having a problem where the new code in the
  application writes to the log, but the code in the old library does not write to the log. I even have one class
  which is a subclass of something in the old library and is also called by code in the old library, and this code
  also will not write to the log. I have configured the application to use log4j2 .

• How can I get the code in the old library logging into the same file as my new code?