I have a 15 year old tiled shower over a non-leaking "composite non-slip" pan. I plan to do a medium remodel of the bathroom which will include removing the wall tiles/wall board, and reinstalling "backer board" and new tiles? We'd like to be able to tile over the existing pan. Is this possible? What prep steps are suggested?

Not too sure you could safely do it. Lot of flex in a composite pan. I'd be afraid a wet bed would break up and backer board is hard to configure. I'd pull the pan and do a full wet bed on ply with a elastomeric liner. Maybe someone who reads the post has done something like you propose.