I am thinking about replacing a tub with a shower pan. Can I get a right drain shower pan and connect to the existing 1.5 inch ABS drain pipe? If so is there a kit available? How does the pan drain connect to the existing drain line? Or is it a matter of going from 3 inch to 2 inch to 1.5 inch? Are there shower pans with a 3 inch drain opening? The tub is on the second floor with little access.
Thanks.

Most showers are 2" drains. The trap raiser would have to be increased. However the good news is they make a replacement shower pan to fit a tub.
Click on; http://www.swanstone.com/products/showerWallsFloors/retrofit/index.php
To check it out. Good luck, Tom