Play as a new character on the old map in single player

Solution 1:

After lots of testing, I've figured this out.

Deleting playerdata will not work because level.dat contains your singleplayer player info and supersedes whatever is in playerdata. However, level.dat does not just contain player data, so deleting it will mess up the world. Here's the solution:

1. You need an NBT editor. I used NBTExplorer.
2. Close Minecraft while you edit the world
3. Delete the Player node from level.dat
4. Delete your file in playerdata
5. (optional, if you want your stats removed as well) Delete your file in stats
6. Reopen Minecraft and play in a restarted world

Solution 2:

Jump into a large pool of lava. Your inventory and experience will go with you. A few attempts with the contents of various chests should complete this process fairly quickly (far quicker than posting on the internet and trying out various hacks).

Solution 3:

Generate the world with the same seed as the first world. Assuming the world was generated with a recent version, this should work.