Originally posted by avatar.zero:

In general, a zone (64mx64m block) on the map becomes "created" (landscape and contents calculated and generated) once any part of it comes close enough to be on your minimap, even if that zone doesn't actually get "revealed" on the map. Such zones that were created before the appropriate update will not have any of the new stuff (because they already exist), but zones that have not already been generated have the possibility to (because they don't actually exist yet).

tl;dr: If your existing map hasn't been too heavily explored already, then it's still possible to get the new content by exploring areas you haven't gotten to yet. Otherwise, it's only possible by either starting a new world or using a mod.

+1 totally confirm this i avoided Mountains prior to H&H update and now Moder is in my nearest high Mountain biome with Other goodies nearby... i did compare my seed to an online world seed viewer and yes things in my world moved a bit from the original seed due to my sailing and wandering into some swamps