We have examples in the Koha Manual about Batch Record Modification Templates to show updating specific subfields, but we don't have any example control fields in there. There is a subfield code field. For fixed fields, you have to leave it blank, but there's nothing that says that. (Like 000$, 001$... etc. you would have to leave the subfield code blank) We should add an example of one of these leader fields. Page: https://koha-community.org/manual/latest/en/html/cataloging.html#marc-modification-templates