Signs worth acting on
Movement in the shower floor when you step in or clean it, a slight flex or give underfoot, is one of the clearer signs a pan has started to fail, since a sound pan and subfloor shouldn't shift under normal weight.[1]
Cracks are another direct sign. A crack in a plastic or fiberglass pan means the material itself has failed; in a tiled pan, cracked or crumbling grout at the floor-to-wall joint is often the earliest visible clue.[1]
Water stains or discoloration on a ceiling or wall below or beside the shower, a musty or persistent mildew smell, and tiles on the shower floor that sound hollow when tapped all point the same direction: water is getting past the pan.[1]
A simple way to test
Plug the drain, fill the pan with about 2 inches of water, mark the water line with tape, and leave it for several hours without using the shower. If the level has dropped when you check back, water is leaving through the pan or its drain seal rather than through evaporation alone.
What replacing a pan actually involves
A prefab pan swap, in a shower that already uses a prefab pan, follows the same rough sequence as a full prefab replacement: remove the old pan and walls if they're one connected unit, confirm the drain and subfloor are sound, then set and seal the new pan before rebuilding the walls above it.
Replacing a tiled pan is more involved, since the sloped mortar bed and waterproofing membrane beneath the tile have to be rebuilt from the subfloor up, not just the visible tile surface. This is part of why a full tile shower replacement runs toward the higher end of national installation costs.[2]
Should you replace just the pan, or the whole shower?
If the walls are sound and only the pan has failed, a pan-only replacement is possible, but it means demolishing the bottom of the wall material to access the pan's edges either way, which narrows the savings versus a full replacement.
Most contractors will walk through both options and their relative cost once they can see the extent of the damage, since a pan leak that's been going on for a while can also mean the wall material behind it needs attention.
Don't wait once you suspect a leak
Water that keeps finding its way past a pan doesn't stay contained to the shower. It travels along the subfloor and framing, and by the time it shows up as a stain on a ceiling below, it's often been moving for weeks or months.
Acting on early signs, movement underfoot, a persistent musty smell, or grout that keeps failing in the same spot, tends to mean a smaller repair area than waiting for a stain to appear somewhere else in the house.