If I'm retiling my shower, do I have to replace the pan, or can I just rough up the shower floor tile, and tile right over. I will definitely remove the wall tile, but the pan is fine and a lot of extra work.

You can not rough up floor tiles and tile over them. You can chip and remove the floor tiles without ruining the pan but it will take a lot of work. This is the way to go if you want a correct job.

How old is the pan? What material is it made of?
Let me ask this, if you are rebuilding a car motor, would you change the rings? Why?