Issue with "Synced Flush" for Elasticsearch (ES) V 1.5.2

Yes Christian.. I just saw this in one of the blog post (Replicas synchronization).
Well, if thats the case do we know a best possible way to perform the upgrade to newer version of ES without the "synced flush" process? Or is there a different process of upgrading these older versions of ES..??

Please do add your points..

Thanks
Vikram Y