+1 to dlochner's suggestion of using an oscillating tool w/scraper blade. I never tried using a debonding chemical, but that sounds good. Along with the wedges mentioned you might try a painter's tool or a putty knife to get under the rim by tapping the tool. On a project of mine I removed chunks of the 5200 (or similar) material..... Hard, cracked, pinkish color, etc.
Added: I remember putting plywood between the inner and outer fiberglass in order to fill in the void after the 5200 was removed. Seal the snot out of the new installation.
Added: I remember putting plywood between the inner and outer fiberglass in order to fill in the void after the 5200 was removed. Seal the snot out of the new installation.