I tend to delete my files after a certain amount of time for 2 reasons. 1 is that I just like to change the map and 2 (which is the larger reason) is the worry that when a new version comes out that the file will be corrupted. I've seen people say they've had the same map since beta, and I couldn't imagine that, but the wonderful thing about Minecraft is how huge the world is on PC, you could easily build a town in every biome and just jump between areas depending on your desired look. I played a while ago and I don't remember if there was a new version/release, but I had giant square holes all over my map. It cropped off lots of things including part of my house. That has only happened that one time, I haven't had the issue since, but again I'm always making new maps.
What does happen to maps once there is a new version?
Areas you've explored should stay the way you left them. For instance, if you explored a Tundra in 1.13, that tundra won't have berries in 1.14. The plains north of my island won't have bees when I move to 1.15, but the plains west of my ocean will, since I won't be going there until after rollout.
The main exception is where the seed management engine has had significant changes. This happened between 1.12 and 1.13, causing some worlds to generate strange borders between previously loaded chunks and new chunks.
Yea, I think that was around the time I had huge chunks missing, around 1.12. That really made me worry about putting too much time and effort into doing a lot on any one map. When I had tried to find out more info for this subject it confused me because people were talking about recoding the file or something, and I don't like messing around with stuff like that.
I keep the files forever, hard disk space is cheap.
When a new version is released I create a new profile with its own saves directory and lock the old profile to the old version of Minecraft.
If I want to continue playing on the same world I copy it to the new directory, if I don't I just create a new world in the new directory.
Either way, if I want to take a look at an old world in its original version of Minecraft I just choose that profile in the launcher.
(Don't EVER open a world save in an earlier version than it was last opened in, unless it's a backup, that's likely to make the earlier version think the file is corrupted and overwrite chunks.)
I don't often go back to old worlds, it's more of a "nice to have them around just in case" thing.
When I do play older versions of Minecraft it's usually to check out how the game has changed if somebody is asking in the forums about how to do something in an old version or to play on a server that isn't updated.
At one time I used to delete my old worlds but later on I decided to keep them after I found a backup of many of my old worlds, including the one I'm currently playing on (the backup of this world wasn't the most recent so I decided to play on it until I'd played at least as long as before; currently, I've played it for over double the time I originally spent on it).
As far as updates go, I don't update unless you count changes to the mods that I use and that has become less frequent over time (in late 2013-early 2014 I had half a dozen worlds created with different mods but have since only had 3 more modded worlds); the nature of these mods (of my own making; I do not try to maintain compatibility with older versions so it can be impossible to even update without corrupting worlds), the desire to keep worlds free of world generation changes (in some cases, quite extreme; for example, one of of my mods increased the depth of the ground by up to 128 blocks), and to have everything that I've done in particular mod confined to a single world are reasons why I make a new world for each mod.
Also, once I stop playing on a world I usually don't return to it later, with the exception of my first world (played on for four stretches lasting up to 15 months, compared to about 5 months for other worlds) and my current world (originally played on for a bit over 3 months, currently 5 months since I started playing on it again). As mentioned by Hexalobular, I also make a new profile and game directory for each world, partly because 1.6.4 doesn't save statistics per world so this is the only way to separate them (though as mentioned above I generally only play on one world at a time).
A few of my older worlds are a bit problematic if I wanted to return to them since I used a backpack and an armor/tools mod on them which I no longer have (the version of Forge I used wouldn't even run on modern Java due to a bug) and one is no longer available due to the GDPR; I'd have to edit them to move the items in the backpacks into chests so they wouldn't be lost and add in replacements for the armor and tools (I later on modded in my own items based on the original mod, but with different properties), and replace all instances of their ore in the world with a block ID below 256. Even then, in one case I made changes to the underground generation separate from the version I played with and don't have the original (I consider this to be in the same category as changes to surface terrain/biomes).
I keep all my old worlds but I don't update them. When a new version comes out, I start a new world for it. I just create a new Installation for the new version but keep all the old installations also and go back to play the old worlds when I feel like a touch of nostalgia.
Inside the later versions they're separated into sub-folders of creative and survival. The rest is on par with what Hexalobular has said, storage is cheap and I can go back in the past via the launcher or keep to the present day with the game launching from a different hard-drive should I want to go further back or into future snapshots.
I DO delete worlds that I only visit to check out some unusual feature that someone mentions in the Seeds forum.
Otherwise I keep all worlds I've spent any amount of time in.
I have defined Game directories for each major Java MC release. ie 1.12, 1.13, 1.14, 1.7, 1.8 etc.
That way I don't corrupt a world with a different, incompatible MC release.
I have 50 profiles defined in the Launcher Including 9 Optifine profiles and 1 Forge profile I created as a test.
Last time I counted I had over 800 MC world folders dating back to mid 2012 when I bought a new computer system to replace the one whose HDD crashed big time.
I now run on two 500 GB SDDs and have 4 1TB HDDs set up as two Microsoft 1TB storage spaces (100% backup of all files)..
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There are no dangerous weapons. There are only dangerous people. R.A. Heinlein
If you aren't part of the solution, then you obviously weren't properly dissolved.
The Meaning of Life, the Universe, and Everything.
Forests, snowy forests
I always liked the idea of having a "permanent" world that i can explore and build on endlessly but new cool stuff keep coming into the game.
I had an idea since before the aquatic update, but decided to wait for it to hit before i started working on the idea of a "permanent" world.
So when the aquatic update hit i spammed random amidst and mineatlas seeds till i found a very large island seed in the middle of an ocean.
The idea is that i make a very large base (100x100 to be more precise), and fully explore the island and its shorelines (got a 2x2 map on the wall in my base looks pretty cool).
From this island i can make a nether portal hub and hop into any place in the world far away from the island, and eventually bring back all the stuff and resources i want (like the new berries that came last update)
This island is now generated and permanently saved and will never change and should never bug out due to new updates so i can keep building on it endlessly.
And when a new patch comes out with lets say a new biome, or a new animal/plant that spawns inside a specific biome, i can use the portal hub to teleport to those new features instead.
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I don't like giving too much info about the seeds i share. I feel like its better to let others explore and find out the chest contents,dungeon places and biomes. If someone is giving me a gift i would rather not know what it is until i open it for my self.
I make a new world all the time as I spotlight mods (as a player it depends but as a Youtuber I sometimes create more just to compatible with what I'm working with leads to more worlds) but otherwise if I have too many or it's been a few months/a year and it's not relevant I'll just delete it. That and loading up more just makes it tougher to open the singleplayer world select so even more reason to. XD Same with instances when using a launcher like MultiMC, I have the hard drive space sure but I noticed it a lot more when I upgraded my PC and had to move my stuff over so I've cut down on what jar files from mods and worlds or instances I have (I don't use many resource packs/shader packs).
If it's an old version I want to go back to I'll keep it or if I plan to remake something like I am with my retrospective mod series 'history of mods' and showcasing the mods/needing the links in descriptions of the videos then I'll keep them around but usually it depends.