Well, for Ivies, say you want stem and they will review you in that context- the courses, rigor, performance, scores and ECs. They want to see you can hit the ground running and have built the mindset through the right experiences. At the same time, most top colleges like depth and breadth.
If you say, eg, that you want classics, but your whole background points to stem, they can certainly question that. In ways, it’s not about the fact that kid can change majors, but their thinking at the time they apply, the awareness and logic in what they do choose.
You want to see, in addition to the individual college’s supplement, whether they ask for additional supps for certain stem majors. And you need to know enough about the targets, to know what your best presentation is.