You can by getting nbtexplorer, then navigate to where your save is then in level.dat change hardcore to 0. I've had to do this a couple of times simple because I don't feel that I should be punished when a mod (sync) is bugged and pushes me in to a wall/kills me when I'm transporting to one shell to another.I have a question.
Is there any way to make a normal crash landing world but in survival mode???
I hate hardcore...
ALSO BEFORE YOU DO THIS MAKE A BACK UP OF YOUR SAVE, THINGS CAN ALWAYS GO WRONG EVEN DOING THE MOST SIMPLEST OF THINGS!