Modify Instances in the WebI History

Help,

I need to modify the layout of an historical instance, in order to make possible the export of the instance in excel format.

Is there a way to save some layout changes of an instance, in the history of the report?

I cannot use the "Save As" option as I don't want a new report but I want to modify the historical instance or to have a new instance in the History.

Thanks for your precious advices!