What changes in Miss Brill is her self-perception, a perception symbolized with the fur necklet that she inspects after pulling it from its box.
"'What has been happening to me?' said the sad little eyes." And, after looking fondly at it, "something light and ...gentle seemed to move in her bosom."
Miss Brill has been "looking through a glass darkly," perceiving herself as an integral part of those gathering at the Jardin Publique for the Sunday concert. She has distanced herself from those older couples sitting silently, and
they looked as though they'd just come from dark little rooms or even--even cupboards!
All the time, however, she has been a part of these silent people, for in the ending of the story, Miss Brill passes the baker's and climbs the stairs to her "little dark room--like a cupboard"--and sits for a long time. Finally, she unclasps the necklet, without looking at it, for it resembles the old ermine torque on the head of the rejected woman at the park. When she puts the lid on it, Miss Brill hears crying, the moaning of her heart, for she knows that she can never again go to the park on Sunday for the concert with the same spirit.
but now she sees clearly after overhearing the young couple who sit in place of the older ones.