There should be a way to update older worlds to newer updates with new/changed biomes and naturally generated world features even if the chunks have already been loaded in older versions (e.g. updating older oceans to new).
This should NOT replace builds that the player has created. This could be done in a feature that allows the player to select which chunks are/aren't updated.
- This could even be done (if possible) by automatically transferring builds to a newer seed.
- Or by having an external program allow the person to select what areas get updated.
There are so many new biomes and biome features coming out now that older worlds become too boring to play compared to newer ones with the new features.