I'm going to keep this simple.
So, I started a hardcore world recently, and I've grown attached to that world, playing it every time I go on SP. I don't want the world to be deleted if I die, because I've made a nice house, tamed a few dogs, and have a good amount of iron. I'm just wondering, is there a way to change my world from hardcore to survival?
Create a copy of the world set to survival mode and copy and replace all files from the hardcore world manually in the game folders, except the one on difficulty (probably player.x).
I copied level.dat from the new world to the save of the hardcore world, it seemed easier, and as far as I can tell it's worked.
It changes the players position and the time of day.
It seems that it would also replace info about whether the dragon was alive and how many end city portals had been activated and I'd store anything in your inventory that you wanted to keep in chests.
A quick glance at the level.dat Wiki page didn't indicate anything else that I recognized as important (apart from world specific stuff like the seed that you'd set the same when creating the donor world).
But I don't really have any idea what to look for so it should go without saying that you should backup your world before trying this.
I would highly advise NOT to just replace level.dat because of all the important data it stores about the world (e.g. as Hexalobular mentioned); it is much safer to use NBTExplorer to change the setting (if you don't like the idea of downloading a 3rd party exe file I can say with certainly that it is safe and quick and easy to use, no installation required and it is compatible with any version).
If you do end up dying, if this is a single player world, you can pause the game, set it to a Lan game with cheats on, respawn then relog and you'll still be in hardcore mode but be alive again with all of your valuables :))
I know this is a REALLY OLD THREAD. But today, I figured out how to do this so I want to share the knowledge with others who might search and find this question-
I wanted to play a Hardcore skyblocks map but in regular survival.
I downloaded two maps- One survival, one the hardcore world that I want to play.
Went into the Hardcore Map game folder- Into the Region folder. Copied the r. files there.
Went into the Survival Map game folder- into the Region folder. Deleted the r. files present and replaced them with the files I'd copied.
When I open the Survival Map, it's a normal survival game- but with the islands from the Hardcore world.
I believe this would work with an established map- But I would strongly recommend making a backup before making any changes to your region folders.
As mentioned a few comments up you do not need to mess around with the game files in this manner, which can cause other issues (for example,the game uses level.dat to record the progress of the Ender Dragon fight and the locations of End gateways) - use NBTExplorer to change "hardcore: 1" to "hardcore: 0" (that is to say, change the tag's value from 1 to 0; example of level.dat).
Also, as I mentioned before replacing level.dat on an established world means you lose all your player data (inventory, XP, ender chest, respawn point, etc) - due to the immense complexity of modern versions (a list of everything saved to level.dat) I'd really never want to replace game files like this (you also didn't mention that you must save the seed and any custom world settings).
ETA: the game also saves entities and "points of interest" and other stuff separately from chunk (region) files, so you'll also get really odd mixed-up worlds if you only replace one set of files (as I said, the save format has become exceedingly complex over the years, no longer just some basic settings in level.dat and a handful of files, this also makes worlds much more prone to corruption due to crashes, etc).