A standard alcove tub occupies about 1,525 by 760mm and leaves a very specific hole when it goes: a long narrow opening, a strip of bare subfloor, three walls tiled to roughly 1,500mm above the floor and painted above that, and a waste at one end. The instinct is to build a shower in the shape of what left, and it is worth resisting, because 760mm is a narrow shower.
What to do with the footprint is the first real decision. The 1,525mm of length is generous, more than a shower needs; 760mm of depth is tight, and 900mm is where a shower starts being comfortable. So either the depth comes from somewhere — moving the front wall 150mm into the room where it can be spared — or some of the surplus length becomes a bench, a full-height storage wall, or a cupboard opened to the hallway behind. A 1,200 by 900 shower with a bench beats a 1,525 by 760 corridor.
The waste almost always moves, and how hard that is depends on which way the joists run. A tub drain sits at one end against the wall; a shower wants its drain centred, or a channel along one side. Relocating means opening the subfloor, re-running the trap and arm with a consistent fall back to the stack, and re-venting. Joists running the same way as the alcove let the pipe travel along a bay freely; joists running across mean drilling within the limits that keep a joist sound, or working from the ceiling below. That is a five-minute look with a torch, and it changes the scope.
Then there is a band of wall that has never been wet. A tub surround typically stops about a metre above the rim — a metre and a half above the floor — and everything above that is painted drywall. A shower is wet to above head height, so substrate, waterproofing and tile continue to around 2,000mm, and the old junction gets opened rather than tiled over. The returns at the open end, which the tub used to shield, come into the wet zone too.