rely on "grandfather rights" for certification
It's not "grandfather rights". The airplane presented for certification to the FAA will be presented one of three ways: A brand new design, to the latest design requirements, A derivative of a type for which the manufacturer holds a type certificate, approved to the original certification basis (
very unlikely for an airliner), Or, A derivative of a type for which the manufacturer holds a type certificate, approved to an updated certification basis - most likely scenario. The applicant and the FAA will agree with a certification basis under the "changed product rule (CPR)" guidelines. It is certain that when CPR is exercised correctly, the most applicable and practical more recent design requirements will be applied to the derivative design. It would not always be possible to apply the latest design requirements to a derivative of an older design, so the FAA realizes the need for discussion and compromise. The CPR determination will be documented for future reference.
It will not be a case of "the oldest" (grandfather), or the newest (may as well be a whole new design, for the work involved), it will be a well thought out compromise. Recent history with the 737 MAX and MCAS would suggest that a part of the CPR process was not correctly applied for that change. I'm confident that someone is reviewing that. In the mean time, I have to hope that the need for objective application of the CPR process will be fresh in the FAA's mind these days!