adding new report means you have to change all controller class and all references wherever salesInvoice is being used in whole AX. It's not only printed at time of invoicing.
it is being printed for Performa purpose also. its also being used at Sales Invoice journal.
In addition to that it is integral part of Trade documents.
what was special need to add new report??? why you cannot think of adding a new design in existing report?? are there any challenges?