I want to start a 1.17 world by playing in the snapshots but I am wondering if there are new updates to the snapshot, will my world still get the features from the update? For instance, when the new mountain generation comes to the 1.17 snapshot, if I load new chunks will my world load the new updated mountains or the old ones?
As of this moment, 11;20 EST (GMT -5) 02/19/2021 the current snapshot for 1.17 is 21w07a.
If you open a world that was created with 21w06a (you can't open any earlier worlds, they are disabled) you could find that your saved world is corrupted.
I would suggest (as does the launcher) that you make a backup of any world you wish to open with a newer release or snapshot.
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The only time I know of where an old world did not receive any new features in an update was 1.2, which added a special world type which omitted jungles from old worlds so world generation remained the same as 1.1, and even then you could easily edit the save to change the world type (or even intentionally get it in versions up to 1.6.4):
(as an aside, this world type actually still exists as "legacy_biome_init_layer" and still changes world generation, if not in the original manner; I have no idea why they didn't remove it in 1.7 or programmed it to replace the biome list for "hot" climate zones with the one for 1.1, which means that savannas never generate and deserts are much rarer, while jungles are unaffected as they are "warm")
However, Mojang has never done anything like this since; in fact, they've even added the ability for some features to generate in old chunks, such as ocean monuments (they would even generate in worlds created before 1.7 so you'd get monuments in the middle of a forest or desert since the land/ocean map changed in 1.7 and the game used the current biome layout to place them). I imagine that by the time 1.17 comes out they will have implemented a solution for pre-1.17 worlds, such as removing the original bedrock layer and generating new terrain below, meaning that you can get new features in existing chunks (I don't know what they actually will do but this is certainly possible).
Any portion of the world that has already been generated, will remain same unless you manually delete some region files.
Any newly generated terrain, once you open your world in a new snapshot, will be generated via the new rules. You might see some interesting artifacts at the old vs new borders, such as absolutely flat vertical walls, like terrain was cut off with a knife. And yeah, if a change has been significant enough, your old world may get corrupted on load.
Now, how will it work for pre-1.17 worlds converted to 1.17? This youtube video kinda illustrates the result:
Essentially, whatever part of the world was generated earlier, will have its bedrock at Y=0..4, with air below that and then the void.
While any part generated post-conversion, would have world generation down to Y -64, with bedrock at -64..-60. Which kinda means you could be digging down in the negatives, and break out into the 'older' area and find yourself staring into the void under the old world bedrock.
What they can implement instead?
Easy option, during world conversion, they could just fill any old-gen chunks with bedrock from 0 down to - 64.
More advanced option: generate the below-0 chunk portion as it would in the new version and replace any bedrock above Y=0 with either stone or deepslate.