I need to amend my client's SA100 for 2017/18 but am now using different software. The box involved is not one HMRC will accept changes to over the phone and there is a considerable tax refund due. I've been told there is an 8 week wait if I write in with the correction. Does anyone know if I can re-do the return on the new software and submit without causing any issues? Has anyone done this successfully?