"Being Generous doesn't mean Rarity will have a completely open mind without any doubt.  She still has to have some doubts in the Changeling she was supposed to watch over.  After all, this species has caused her much trouble twice already.  It made her unsure if this scary looking creature could be good.
But once she got to know it better, understanding its reasons for its bad behavior (because it was taught to be one), she started to believe that maybe a Changeling can change after all, perhaps it really was possible to teach one to be good.  It can be taught what was the right and wrong things to do and learn from those experiences.  A Changeling is also quick to adapt to many things, and her Changeling proved this by being able to keep up with her work.  It also learned to be generous like her by helping her and others without asking for anything in return.
 
Well.... Maybe it still got love to feed in return, but that’s acceptable, since it no longer steals it from unwilling ponies anymore."