That was cruel John
I just took my neck off of my 2nd guitar, and I used a spackle knife with an iron. I didn't care about the finish, so I did no protection. But I would do the card board trick if I ever cared about the finish. You should sand the tip of the spackle knife down to thin it, it will be much easier to get under the neck. Since I used hide glue for my neck extension, I wanted to use steam with heat. I put a damp rag on the FB extension then put my iron on top of it and let it sit for about 1 1/2 mins. I then heated the tip of my spackle knife and slid it under the FB extension in front of the soundhole. I worked it side to side while the iron was still on it. Eventually it gave away with no wood being pulled up. You should go real slow, about 20 min total. Do not rush it, and check to make sure the heat is not damaging the finish. Good luck!
Tracy