If you look at prior year returns for a new client (I require them) you have a Circular 230 responsibility to inform your new client of errors and of the consequences of amending or not amending. You do not have to amend. The client does not have to file an amendment. (Depending on the errors &/or the potential client's attitude, I might or might not accept a new client who does not amend.) And, yes, you can charge for amendments. You do not have to audit (as an EA, I can NOT audit). You do have to notify if you know. Our standard is Know or Should Have Known.