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).