On JMX Console the MBean jboss:service=HAPartition,Partition=.... has an attribute JGroupsVersion.

This displays 2.6.10.GA( $Id: Version.java,v 1.59.2.18 2009/04/28 14:25:33 vlada Exp $)

I have updated the JGroups version by removing the jgroups.jar and adding the jgroups-2.6.15.GA.jar.

After the startup the HAPartition MBean still reports 2.6.10.GA

IMHO the problem is that the class org.jboss.ha.framework.server.ClusterPartition uses the class org.jgroups.Version to get the attributes "description" and the "cvs" info in the method getJGroupsVersion().
But the strings in the class org.jgroups.Version are static final and therefore the value is added to the bytecode of the class org.jboss.ha.framework.server.ClusterPartition at compile time. A change of the jgoups version didn't change that.

So org.jboss.ha.framework.server.ClusterPartition reports the JGroupsVersion against it was compiled and that which it is available.

A use of the Method org.jgroups.Version.printDescription() instead should fix the problem IMHO.