I think your understanding is correct. In the past, each fare subclass would have its own full set of "conditions", e.g. K and upward would allow stopovers in both directions and date change free of charge. Now it appears stopovers are still determined by the subclass but date change flexibility is determined by whether the fare is light, essential, or flex. In other words, there is an additional dimension to the fare structure. Perhaps we can start labeling them as VL, VE or VF etc..