When one fixture is swapped for another in a room that otherwise stays the same, the existing hole position dictates what can go in. A basin tap arrives at the new fixture with hole spacing that either matches the existing holes or does not. If it matches, the new fixture is installed into the existing openings. If it does not, the top has to be reworked to enlarge or reposition holes. For floor-mounted fixtures like toilets, the rough-in dimension is fixed. A new toilet with the same rough-in dimension sits on the existing flange. A different rough-in means opening the floor and repositioning the waste.
The footprint of an old escutcheon leaves a mark. A basin escutcheon or a toilet flange sets on the surface and casts a shadow or leaves a dirt ring around it. When the fixture is removed, that mark remains on the wall or floor. A new fixture's escutcheon might be a different size or shape. A smaller new escutcheon does not cover the mark left by the old one. A larger one might cover more than the mark but overhang the finished surface in a way that looks wrong. The size and shape of what sits on the surface matter.
Swapping one fixture makes untouched fittings around it suddenly look old or mismatched. A new shiny tap next to a weathered old mirror shows the mirror's age. A new white toilet against aged tile on the walls reads as a patch, not an upgrade. The fittings that are not changing stay as they are, and the contrast with the new fixture becomes obvious. This is not a problem if the new fixture is intentionally chosen to complement what is there; it is a visual issue if it is simply the latest available model in a different aesthetic.
The point at which a swap stops being a swap is when walls or surfaces have to open. If the fixture fits what is there and the escutcheon covers what is underneath, it is a swap. If holes have to be repositioned or resurfaced, if plumbing has to be rerouted, if electrical connections have to be relocated, the boundary of the swap has been crossed. At that point, it is no longer a straight replacement; it is renovation work.