So unlike the MAX, the X is a new aircraft, with full new model testing, no exemptions (in full or part) to the latest regulations, and no grandfathering?
As has been explained many times in this thread, any new regulations apply only if the part has been changed.
There may be many unchanged parts, identical to ones being delivered on 777-300ER's today, that will not be subject to a new certification on the 777-9.
The issue is complex and subject to many years of discussion between Boeing and the FAA to develop the 777-9 Certification Basis. No simple statement will provide an accurate answer.
Again, H L Mencken made the correct statement for the situation: "For every complex problem there is an answer that is clear, simple, and wrong."
In US building design there is the model code the "International Building Code" that is updated every 3 years. When adopted by each state, the states can add or remove requirements in the adoption, then the locals do the same. I design a building, turning in the design drawings along with the calculations for review. Approval of my project does not remove any of my liability, they are not responsible unless they interfered or changed my design, and they clearly avoid that. I am on the hook for the design for 30 years in Washington State, my E & O insurance for what looks like a paltry sum runs 9-10% of revenue. I can design right to the code limits but that worry about a claim keeps me well away from that. I won't design for any amount of money anything that I feel is unsafe. The FAA's FAR part 25 is the aviation industry equivalent to the IBC. The $ 2B+ that Boeing will be paying is their insurance claim.
In building as well as in aviation all new work is to meet the latest regulations, but the existing unchanged items do not provided that it can be shown that
the Demand remains less than the part original Capacity. But in the regulations new regulations can be made to apply immediately to all cases, apply to cases that exceed say 20% of area or value, or existing cases need to be applied by a certain date. In buildings, after the Great White fire, all nightclub occupancies in most jurisdictions had to have sprinklers installed within 3 years, but upgrading for seismic or FEMA flood elevation only apply when alterations exceeding 50% of value over the past 10 years. The IBC codes went more performance based, where if sprinklers are added the requirements for fire walls, exits, etc are more relaxed than previous codes, but if no sprinklers the requirements well well up. It reduces building cost if sprinklers are added and made the added cost of sprinklers less, so more buildings are sprinklered. However, that didn't really address that in the last 20 years the fuel content of furniture went sky high with all of the foams used.
In aviation, fuel tank inerting requirements are quickly making the 744 too costly to operate. Wing lightning protection is a rule that is I believe fully grandfathered. I am sure there are many requirements that are grandfathered but really shouldn't be, other requirements that are excessive and just add cost or other issues, and items that are not grandfathered but should be allowed.