I have a crappy metal shower with a terrazo shower pan. Replacing it all with a bigger tiled shower. How do I remove the terrazo shower pan. I am thinking of using a sledge hammer, but fear damaging the drain system

If you are 100% sure it is real terrazo and not some faux fiberglass terrazo the sledge is the way to go. Break the sides first inward and outward. Sledge the floor last and stay back from the drain by about 10". You should be OK and much of your drain will be replaced when you create your mudd shower pan floor with a membrane.