https://www.flightglobal.com/FlightPDFArchive/1996/1996%20-%200036.PDF"The latest issue concerns the Boeing 737-X series and whether, despite being a rewinged, re-engined, re-instrumented version of the previous series, it will gain any advantage over the A3 20 by virtue of grandfathering anomalies. This has yet to be made clear."
Apparently there is a grey area. Is an aircraft a derivative of an existing aircraft or is there so much new you should test and certify as a new aircraft.
Aircraft manufacturers prefer grandfathering of existing STC's (also e.g. A346). If you are certifying to today’s rules instead of rules from thirty years ago – today’s rules are always tougher than yesterday’s. But, those rules are tougher for a reason & flight safety usually is part of the deal.
On the 777X:
https://www.federalregister.gov/documents/2018/05/18/2018-10576/special-conditions-the-boeing-company-model-777-8-and-777-9-airplanes-folding-wingtipsOn April 19, 2017 (for the Model 777-8 airplane), and May 12, 2015 (for the 777-9 airplane), Boeing applied for an amendment to Type Certificate (TC) No. T00001SE to include the new Model 777-8 and 777-9 airplanes.
The 777-9 and 777-8 have entirely new wings, engines, cockpits, landings gears, fuselages (structure, higher, bigger windows), control dimensions and many systems. Apparently it is certified as amendments on Type Certificate (TC) No. T00001SE, 777-300ER.
On the 777-9, will FAA / DoT have a second look & check for short cuts / efficiency's in the certification process? Out of the 737MAX research, new certification process requirements might see the light. Inclusion in the 777-9 STC process might depend on the level of 777-300ER grand fathering allowed. I one believes old agreement on this will stand, there might be surprises. Things happened.
New fuselage, controls, tail, wings, engines, new cockpit. Is this a 777X or a 7X7?
Apart from safety considerations, are there economical drivers that conflict?