Currently, Jira does not support setting a default value for a custom field for the non-newly created issues. Custom fields are created with no value, and any default value must be set afterwards. This means that users need to perform additional steps, such as running bulk operations, to set default values for existing issues.

Feature Request:
Add the ability/option to set default values for custom fields for the existing issues. This feature would streamline the process and save time for users managing large datasets.

Use Case:
When adding a new field, users currently need to run a bulk operation on thousands of records to set the default value, since the default value is being added automatically only to the new issues. Allowing default values to be set automatically for the existing issues would eliminate this extra step and improve efficiency.