Does the minecraft server automatically add the new features once updated, even in the old saves, or it only does that in the new games?
It depends what you mean by new features. I can't guarantee how things will go in the future, but here's how things generally have happened in the past.
- New block types will not be added to the existing map. They WILL, however, start appearing in new chunks. This was part of the reason lapis lazuli seemed exceedingly rare after its update, people were looking in previously generated chunks.
- New mob types will spawn in existing parts of your map. They can appear wherever their spawning algorithms determine is "valid," and chunk age isn't factored in.
Those are the only two cases I can think of at the moment. If you had a specific question or wanted an answer about another group I forgot about, I'd be happy to update my answer.