The effect of initializing an MQCD to MQCD_DEFAULT has changed. You might need to change how you set the Version.
In versions of WebSphere MQ before Version 6.0, initializing the MQCD to the macro variables MQCD_DEFAULT or MQCD_CLIENT_CONN_DEFAULT would set the Version field in the MQCD to MQCD_CURRENT_VERSION, which equates to 7 for WebSphere MQ Version 5.3 and 8 for WebSphere MQ Version 6.0.
In WebSphere MQ Version 6.0, MQCD_DEFAULT sets the Version to 6. If you want to take advantage of features of the MQCD introduced in WebSphere MQ Version 5.3 or 6.0, set Version in the MQCD explicitly to 7 or 8, as appropriate, or to MQCD_CURRENT_VERSION. If you intend your applications to be portable between several environments, use a more-recent version MQCD only if all of those environments support that version.