I’m currently doing a barrier free shower for my daughter in her new bedroom suite. Her shower will be 4x6’, but the entire floor and walls will be tile in the bathroom. I have researched the various pans and there are some made specifically for current tub enclosures, just take out the tub, install the pan and walls, then either tile those, or they have wall sections made from composite materials that are sized for a existing tub enclosure. Look up the Schleuter/Kerdi systems, they’re really nice.