The easiest way I've found to transfer your player to another world involves using iChun's Sync mod and MCedit.
First of all back up your worlds. You should always do this before tinkering with the game's files in any substantial way, anyway.
In the world are transferring from, make a clone with a Shell Constructor and pig(s) on treadmill(s), make a Shell Storage (don't forget to activate it with the lever) and use it to switch from your old player (henceforth we'll call him transferPlayer) to your new clone. Pause the game and click "Save and Quit to Title".
Open the world you were just playing in MCedit and then find the Shell Storage with transferPlayer in it and export it as a schematic (include a few extra layers of blocks on the side the lever is on, as the lever must be included with the shell storage).
Open the target world to which you want to transfer transferPlayer in MCEdit and place the Shell storage containing him. Save and close the world. Exit MCEdit
In the world where you placed shell storage in the previous step, create another Shell storage and switch from the old player from that world to transferPlayer. With that, your player should have been successfully transferred.
Lastly, turn off the power to kill the original player from this world. This way, when you die, you respawn and don't go back to original player for this world.
Done with MCEdit-0.1.7.1 and Sync 2.2.3.