The issue was a bit misguided since I thought I had to convert my existing securityLabelDownloadAckResource.rbInfo into securityLabelDownloadAckResource.java for my Windchill13.0 Upgrade from WC12. The following is the errors that I got after an upgrade to WC13.
ERROR [main] wt.access.configuration.SecurityLabelsHandler - The security label value downloadAckMessageKey: TRUE_DownloadAck; is not a valid resource bundle key. Setting the value to null.
Somehow, building of the securityLabelDownloadAckResource.java in <WT_Home>wtCustom\wt\access\configuration with "ant -f bin/tools.xml class -Dclass.includes" failed to recreate the class in the <WT_Home>codebase\wt\access\configuration folder. Eventually, I had to update the corresponding java file in <WT_HOME>src\wt\access\configuration. Then, build it with <wt_home>ant -f bin/tools.xml class -Dclass.includes=wt\access\configuration\securityLabelDownloadAckResource.java -Dclass.force=true, which worked to recreate of securityLabelDownloadAckResource.class.
The class definition of the java file is:
public class securityLabelDownloadAckResource extends WTListResourceBundle {
@RBEntry("TRUE_DownloadAck")
@RBComment("This is a Sample Download Ack Message")
public static final String downloadAckMessageKey = "TRUE_DownloadAck";
}