We have installed JBoss 4.0.4 which uses log4j 1.2.8.
Our applications are compiled against 1.2.9 and when deployed to the JBoss AppServer we get a classnotfound exception due to the RootLogger class not being present.
We cannot downgrade our own version and nor can we change our usage back to the deprecated RootCategory. We can however upgrade log4j1.2.8 to 1.2.9 in the server lib folders of JBoss. Would this be safe as the following change notes seem to suggest so.

LOG4J Change History:
The following shows the difference between both versions.

- Release of version 1.2.9

Log4j version 1.2.9, is identical to version 1.2.8, except that
several key methods have been deprecated in preparation for version
1.3.0, the next major release of log4j. These changes are intended to
enforce the rule that client code should never refer to the Category
class directly, but use the Logger class instead. Similarly, client
code should not refer to the Priority class but to the Level class
instead.

For a more detailed discussion, refer to the document entitled
preparing for log4j 1.3 at:

http://www.qos.ch/logging/preparingFor13.jsp

February 19th, 2003

- Release of version 1.2.8

- Fixed bug #11570 whereby XMLAppender would throw a
NullPointerException if the input message was null. Many thanks to
David Vandegrift for reporting the bug and to Hendrik Brummermann for
supplying the patch.

• 
  
  - Fixed bug #12366 whereby various versions of Xerces would not parse
  log4j configuration scripts expressed in XML format.
• 
  
  - Fixed bug #14827. The "removes" buffer used in the flushBuffer() method
  of JDBCAppender is now cleared, solving the memory leak. Thanks to John
  Landers for reporting the bug and suggesting the fix.
• 
  
  - Fixed bug #15599. SocketAppender now honors ReconnectionDelay of 0.
  Many thanks to Scott Schram for reporting the bug and providing the fix.