Re: replacing a leaky shower pan
1) will I have to open up the drywall ceiling on the first floor below this shower pan to safely cut the old PVC pipe when removing the existing shower pan?
Yes, although you may be able to cut it from above when you use the sawzall to remove the pan it will have to be opened to make the new connection.
2) I plan to carefully use a Sawzall to slice up and remove the old pan. Is this the bed method or is there another.
If you are removing the tile and wallboard you may just be able to lift the pan out after you cut the drain pipe.
I don't favor cast iron pans in a shower because they can chip if you drop a bottle of lotion or shampoo. I have never used a poured concrete pan so I can give you no opinion on them. I personally like the fiberglass but you can not use abrasive cleansers or it will dull the finish but it seems to be less susceptible to cracking or chipping than acrylic and fells a lot more substantial.
I would use the compression type drain connection. That way it can be replaced if it leaks. In either case you will have to make the connection with access to the bottom of the pan when it is installed.
I should tell you that I am not a professional plummer, but have remodeled 4 houses for myself and I do all my own work and as such the above is just my opinion based on personal experience and research.
Be sure you live your life, because you are a long time dead.-Scottish Proverb