Sync Existing Issues feature allows you to trigger synchronization for issues that existed before the plugin was (or its listeners were) enabled on your system.
You are expected to enter a valid JQL. When triggered, the plugin will act as if an update event was fired on each of the issues returned by the JQL. All related listeners will respond to this event.
You should provide a JQL that will return only the Source issues for sync. Your configured listeners will respond and will find the target issues for each source issue accoring to their configurations.
If your JQL returns both source and target issues then both issues will be processed as a source issue. This might produce unintended results (especially for listeners with bidirectional configuration) as the listener might sync values in one direction first and the other direction afterwards.
The sync process might take a while to complete based on the number of issues returned by your JQL.
The sync screen will show the progress of the sync.
After the sync is started, you can cancel the operation by clicking the "Cancel" button. Cancelling an operation prevents sync on issues that are synced yet but you cannot undo the changes to issues already done.
Comment and Watcher fields are not synced during "Sync Existing Issues" operations.