The "changed product rule," or 14 CFR 21.101, does require changed areas and areas affected by the change to step up to the latest rules as of the date of application for the change as a supposed default, but it has HUGE loopholes in it that are regularly exercised to allow reversion to earlier versions of the rule based on impracticality or an argument that little benefit is provided by stepping up because the unchanged area still doesn't comply. (An example of this would be that it doesn't make much sense to put in three rows of 16 g seats if the rest of the seats are 9 g seats.) The negotiation paper regarding the certification basis for a major derivative transport airplane program ends up being dozens of pages long with all the arguments for reversion to earlier rule versions. Some of these reversions make sense, others are questionable compromises.