This article explains what versions of Jira can be synchronized with one another using Backbone Issue Sync.
Server ↔ Server
Backbone Issue Sync supports Jira Server versions 7.0 and higher. All supported versions can be synchronized with each other – e.g. you can synchronize a Jira 7.0 instance with a Jira 8.x instance.
We suggest to read the related example use cases, e.g. Bidirectional synchronization.
If you are using Jira 6.3 or 6.4, we strongly recommend that you upgrade to a current Jira version since Jira 6.3/6.4 have reached their end of life in July 2016/March 2017. If you still want to synchronize with such a Jira, you can install Backbone 3.9.2 on that instance and configure the synchronization in another instance which should be a supported Jira version. Please note that we don't explicitly test this scenario, so future updates may break this compatibility.
Server ↔ Cloud or Cloud ↔ Server
All supported versions of Jira Server can be synchronized with Jira Cloud and vice versa. In this case, a synchronization needs to be configured on the Jira Server side. Please read this Synchronize Jira Server with Jira Cloud which explains the initial setup steps and best practices. However, please note that distributed configurations are not supported.
Cloud ↔ Cloud
Also Jira Cloud instances can be synchronized with each other. This is documented in the cloud section of our documentation. We suggest to read the related example use cases, e.g. Bidirectional synchronization.