Rebecca started out as a very vein, careless person. She was very selfish and spent money all over the place. But as the story is progressing she is turning less selfish and isn't spending as much money. She's trying to start a new life style and save money to be more successful and get herself out of debt. At the beginning of the story Rebecca lied and said that she needed money for her "sick aunt" and took money from a coworker to go and buy a new scarf. Now I can't see her doing that because she wouldn't have wanted a $300 scarf, and also because she is now only getting things she absolutely needs.
Since Rebecca's change she hasn't really been around a lot of people. She's only been around her room mate so far and she hasn't acted any differently. But at the beginning Rebecca acted almost as she was better than every one else. She is changing for the better, into a better person.
Rebecca's change affects the plot because the main problem in the book is her shopping addiction, and she is now changing to get rid of that problem, which is major in the book.