Summary: The Doctor may be used to regeneration, but this is new.
The Doctor studies the face reflected in the mirror; it’s changed, unfamiliar, new, female, which is odd. The craggy face and grey hair are gone; this body is younger, shorter, and the wrong shape.
Being a Time Lord means becoming accustomed to changing face, body, even temperament and personality; that’s what regeneration does. Old body dying? Have a new one.
On one level, the changes aren’t a surprise, only the Doctor had never changed gender before to his… her knowledge. Always knew it was possible, but…
So here she is, new body, new parts, same old Doctor.