I was under the impression that Marianne begins to change later, but this reading I noticed that a change has already occured.

CH 37;
Marianne at last learns to control her feelings and not give in to her sensibility, when Elinor asks her to behave in an uncharacteristic manner for her, regarding all things to do with Edward and the engagement.

In fact it provides a few LOL lines :)

"She performed her promise of being discreet to admiration. She attended to all that Mrs. Jennings had to say upon the
subject, with an unchanging complexion,.." :)

"... dissented from her in nothing, and was heard three times to say, "Yes, ma'am." ;)

"She listened to her praise of Lucy with only moving from one chair to another,.." :D

"..and when Mrs. Jennings talked of Edward's affection, it cost her only a spasm in her throat.

"Such advances towards heroism in her sister, made Elinor feel equal to anything herself." LOL!!

Later in CH 38 ;

"...in trying to converse upon a topic which always left her more dissatisfied with herself than ever, by the comparison it necessarily produced between Elinor's conduct and her own.