You "have to" tell your client that amending will save him taxes and give him an estimate of how much tax it would save him, as well as an estimate of how much your fees will be to amend.

After that, it is the client's choice whether or not to amend.  With a full step-up, it is likely that amending will save the client more tax than what you would charge for your fees, so the client will want to amend.