so, the 1987 shower walls were tile over drywall.. mildew, mould, and other fun.

I have the walls down to the studs and starting to rebuild. The old shower pan looks to the naked eye to be fine, no cracks or significant wear. Should I replace this as long as I'm in there ? or would it be safe to leave the old one ?
I'm nervous about my ability to install a new pan without getting creaking or cracking down the road somewhere..
tia, Doug