I've never taken apart a neon, but a heat gun will be my choice. I'd start at one end, and get a small part loose, and put some wedges in so it doesn't re-glue itself. Then do the same at the other end, and work your way all around it, so as not to get it real hot in the same place. A temp controlled heat gun would be best, since over heating would be an issue. Some of the sealant will most likely stick, and I'd use the same method slow, a little at a time. Warping would be my biggest concern.