I tore out an old tiled shower and pan and need to replace it. There is a drain in the floor - metal screwed into the pipe coming out of the floor. From the side, it looks like a shallow dish (like a cup saucer) then a smaller straight lip and then the screw-in drain. In my recent research on installing shower pans, they all use the new 3-piece drain with weep holes. How do I replace the shower pan keeping this old setup in place? I'd rather not tear it out. I don't know if it had weep holes when it was put in back in the 1970's-'80's.

You will most likely have to replace the drain as well. You can try to unscrew it with large pipe wrench - or cut it below the drain at the pipe - or take sawzall and carefully cut the drain into several pieces.