One way to do this, which doesn't require MCEdit (which won't work in 1.13 or later and has been discontinued) is to copy the region files from the worldname\DIM-1\region folder to the worldname\region folder, though the rest of the world will continue to generate normally and if you die you'll respawn on top of the bedrock ceiling. For 1.13+ you could also make a Buffet world with the biome set to set to the Nether and using the caves world type, which is basically the Nether, although I don't know if it will be exactly the same; at least in older versions a Superflat world set to Nether wouldn't generate any of its decorations since they were part of the dimension, not biome. You'll properly spawn below the ceiling though, as otherwise the game spawns the player on the highest solid block (unless that has also changed; world spawn was previously always set as (x 64 z) in level.dat, where the actual y-coordinate the player spawned at was the highest solid block).
im trying to turn the overworld into the nether to troll my friends when they enter the netherportal.
using mcedit or anything
One way to do this, which doesn't require MCEdit (which won't work in 1.13 or later and has been discontinued) is to copy the region files from the worldname\DIM-1\region folder to the worldname\region folder, though the rest of the world will continue to generate normally and if you die you'll respawn on top of the bedrock ceiling. For 1.13+ you could also make a Buffet world with the biome set to set to the Nether and using the caves world type, which is basically the Nether, although I don't know if it will be exactly the same; at least in older versions a Superflat world set to Nether wouldn't generate any of its decorations since they were part of the dimension, not biome. You'll properly spawn below the ceiling though, as otherwise the game spawns the player on the highest solid block (unless that has also changed; world spawn was previously always set as (x 64 z) in level.dat, where the actual y-coordinate the player spawned at was the highest solid block).
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?