We have made some significant updates to our CODEOWNERS file which are correctly reflected in the UI, but we'd like to apply those rules to all existing unresolved issues. Is that possible?

Currently, Sentry does not reprocess existing issues against updated CODEOWNERS rules. Any changes you make to your CODEOWNERS file will only be applied to new events that arrive after the update.

That being said, you can manually assign existing unresolved issues without waiting for reprocessing.

We have an open feature request about this on GitHub that you can find here.