Index: wp-includes/capabilities.php
===================================================================
--- wp-includes/capabilities.php	(revision 19130)
+++ wp-includes/capabilities.php	(working copy)
@@ -724,6 +724,7 @@
 		if ( 1 == count( $this->roles ) && $role == $this->roles[0] )
 			return;
 
+		$oldroles = $this->roles;
 		if ( !empty( $role ) ) {
 			$this->caps[$role] = true;
 			$this->roles = array( $role => true );
@@ -733,7 +734,7 @@
 		update_user_meta( $this->ID, $this->cap_key, $this->caps );
 		$this->get_role_caps();
 		$this->update_user_level_from_caps();
-		do_action( 'set_user_role', $this->ID, $role );
+		do_action( 'set_user_role', $this->ID, $role, $oldroles );
 	}
 
 	/**