Alcove tub to shower
The standard hall-bath tub between three walls. The tub comes out, a shower pan goes in the same footprint, the valve moves up, and the walls get new tile or panels plus a glass door.
The drain usually stays close to where it was, which keeps slab work small. Most of the decisions are about threshold height, niche placement and glass.
Garden tub to walk-in shower
The corner tub on a platform in a suburban primary bath. The platform comes out, the drain often moves several feet, and the old tub and small shower are frequently combined into one larger walk-in.
This is closer to a partial remodel than a swap. Our walk-in shower installation page goes deeper on curbless and low-curb builds.