I think Mary and Colin´s transformation is gradual. Mary and Colin both become different people. Mary grows up and is less spoiled, and Colin outgrows his mean ways when he goes outside and sees the secret garden realizing he wasn´t injured as previously thought. As for evidence I am currently re-reading the book now.
The context clues suggest this by stating the demeanor of the Bernard is gentle and friendly and NOT rowdy and the opposite of gentle and friendly would be wild.
