I recently created a survival world for fun. We spawned on a tiny island next to a Mushroom Fields biome. It is amazing. We haven't left the Mushroom Fields, but we have been mining it like crazy and have found three dungeons, an abandoned mine (with no mobs spawning in it, it was eerie), we have many stacks of iron and coal and redstone and other resources etc. We have a portal to the nether and are basically playing in it so we can scratch the exploring itch without loading other biomes that surround us.
My question is this: The internet is full of crazy information. When the 1.14 update hits, will my world get stupid? Will snowy mountains, for instance, spawn in a desert? Will the work we have done been saved? I know "Minecraft is Minecraft" and screwy stuff happens all the time, but I'm just making sure that the world we have spent so much time working on will not be negatively affected by the new update. I can see a forest and swamp in the far distance, but we have avoided them completely to allow villages and so on to update properly. How will new biomes be placed in the seed, which more or less maps the biomes upon starting?
As long as the gap between versions isn't large (for instance, 1.12 to 1.14) and the new version doesn't SPECIFICALLY say anything about changes to world generation, biomes and the terrain should be fine. Usually the only thing affected by small jumps in versions are redstone contraptions, and maybe mob spawner rooms.
1.14 has been known to be very laggy, but I'm sure they will iron that out upon release.
But it wouldn't hurt to make a backup before updating since you typically can't go back, at least not without problems, so if you should happen to not like 1.14 it would be nice to have a copy of your world that hadn't been updated so you could continue to play it in whatever you are using now (1.13?)
New fixed features like biomes, villages, etc will probably only be generated in new chunks that haven't been loaded before, so the areas you can see in the distance wouldn't have new features but the areas further away that you haven't been close enough to to have loaded will have the new stuff.
(When ocean monuments were added they made an exception, they were generated in old chunks if the player had spent less than 5 minutes (I think it was) in them.)
So, no, snowy mountains should only generate in appropriate biomes even if the generation rules are changed.
Changes to mobs should apply to newly spawned mobs in both old and new chunks.
The upcoming changes to villager behaviour may or may not be applied to old villagers.
Would you mind disclosing the seed you are using? I've never seen a mushroom fields biome :-)
Your explored area should be fine, but if you want peace of mind now, just make a copy of your game and load it from within the 1.14 snapshot.
I found a great tutorial in these forums describing how to set up separate game folders for different versions of the game so there is no danger of opening a saved game in the wrong version by mistake. I didn't save the link but it involved editing the preferences for each game version in the Minecraft launcher. You can manually enter a different path for saved games for each version you play.
I had a look at the seed and the mushroom fields biome is very interesting. I love the way it looks. I would love to know how you managed to spend so much time mining there. Where did you get wood for making tools and torches if you didn't leave the mushroom field island?
Very close by is a forest, and we chop/plant/chop/plant the very first few trees in the biome. The rest we have mined for in the abandoned mine shaft, branch mining the bottom, etc. There's even a slime chunk in it.
Even large jumps in versions should not cause any issues; world generation has not changed much since release 1.7.2, over 5 years ago (1.13 only changed oceans, with minor changes to some "M" biomes), and even the snapshots should have no issues loading a world that old, or even older (some people say that you should load a world in every successive version but only the spawn chunks and chunks within render distance of the player are loaded (even in 1.13 unless you use the "optimize" feature), and otherwise it would be incredibly tedious to travel around to load every chunk, especially on a world like this, or one where you didn't map out every place you explored, so the game must be able to handle chunks last saved in every version since 1.2.1, when a complete conversion was required from an older save format, so no older chunks will ever exist in any world loaded in that version or newer).
At worst, you'll get sudden changes in terrain as you explore new areas (for pre-1.7 worlds and oceans in pre-1.13 worlds) and/or cut-off structures (only ocean monuments are able to generate in existing chunks, others will just cut off along old-new chunk boundaries, including the new pillager outposts and villages, where you may also find villages that are half the old style and half the new style) but existing chunks will not change.