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Clicking the Bulk change issues... button will redirect the user to the Bulk Operation screen. This allows you to edit, move, transition, delete, watch, or stop watching multiple issues in one operation.
Bulk change limits
In order to prevent the server overload, by By default, Jira limits the maximum number of issues that can be bulk changed to 1000 to prevent server overload. This limit can be modified by editing the configuration settings or your Jira instance. The property limiting the number of issues that can be bulk modified is jira.bulk.edit.limit.issue.count.