I really wish this would be possible!!! But at the same time judging from all previous EA practice I'm 100% sure this feature will be available only in the new world. No retroactive changes will be made. My hopes here are only for mods. But unfortunately not everyone can/want use them.
More integration of new features with packs would be really appreciated. But I think they already have so many they do not even try that much. Like they hide whims instead of adding new. They never return to the question of swimable water in older worlds. Today in IronSeagul video he mentioned that sentiments are generated only by basegame stuff and new pack all other packs excluded and so on... So I'm afraid not.
