Upgrading from log4j 1.1.x to log4j 1.2.x
Log4j version 1.2 is intended as a drop-in replacement for log4j 1.1.
Thus, upgrading from log4j version 1.1 to 1.2 should be trivial. However, some key classes (org.apache.log4j.Category and org.apache.log4j.Priority) were deprecated (but not removed). In the vast of majority of cases, you should be able to drop in the latest stable log4j 1.2.x jar, and be ready to go without even needing to recompile.
It would be a good idea, however, to replace references to org.apache.log4j.Category with org.apache.log4j.Logger, and references to org.apache.log4j.Priority with org.apache.log4j.Level. This will remove deprecated warnings during compilation.
ToDo: upgrading for cases with custom extensions to log4j