I have a potential new client who has filed at least 3 years with the accrual box marked on Form 1120-S but accrual to cash adjustments were made and the 3 years of returns were filed on cash basis.  To top it off, the M-1 and adjustments in the return were backwards.  I would like to amend the returns but I don't know how to prepare them based on them filing incorrectly for 3 years.  Do I amend and keep it consistent by marking the accrual box and correcting the accrual to cash adjustments for all 3 years?  File Form 3115 on an amended 2022 return and do the same for 2023 and 2024?  I know the IRS can not accept that form as well.  The basis schedule will also have to be updated and that makes me want to go back to when it was possibly filed correctly and recalculate it.  Any ideas you have would be greatly appreciated!