I just love this technical talk and could do it endlessly. In the low pitches the tension in the vocal cords is very low and some singers find it difficult to achieve very much intensity in that part of their voices. This can provide a reason to move to a higher Fach. This is a personal decision.
Now that we are hearing Placido Domingo occasionally sing baritone, we can see why he would want to switch to tenor. His voice is just a lot more interesting up there.
We may well think that Cecilia Bartoli is a soprano, but we might have a hard time coming up with a reason why she should reclassify. When I was obsessing over this subject all those years ago, I was worrying about her, that she might do herself harm. My impression today is that she takes very good care of her voice and there is no cause for concern. There may never have been.
This is the most soprano I have ever heard Cecilia. I think it's this one: Salieri: La Fiera di Venezia - Vi sono sposa e amante. Judge for yourself. As a soprano she would have a much different, much more orthodox singing career.
As it is she sings whatever she wants, no matter how high or low, almost without reference to Fach, and makes millions. Who are we, who am I to second guess this?