When you perform a bulk change, as of now, one cannot undo this.

https://community.atlassian.com/t5/Jira-questions/How-to-undo-bulk-changes/qaq-p/579482

https://community.atlassian.com/t5/Jira-questions/Can-you-undo-a-bulk-change/qaq-p/912234?utm_source=atlcomm&utm_medium=email&utm_campaign=immediate_general_reply&utm_content=topic

It would be helpful to undo these changes. Or at least have JIRA keep a history of bulk changes, say in a customized query, that was performed so you track the issues that were impacted.