Eddy's redemptionThe movie, and the series in full for that matter, is Eddy's story and his road to becoming a better person. While Ed and Double D also need to grow "a brain" and "courage" respectively, it is Eddy who needs to grow a "heart" by owning up to his past abusive behavior toward his friends and break off the abusive relationship he suffered at the hands of his older brother.
Up until the finale, what little we see of Eddy's home life was that he didn't receive much attention from his parents aside from the occasional punishment and that he learned everything about scamming from his brother when he was not being emotionally and physically tortured by him. In other words, Eddy was raised to not trust the people around him and that the only way to get ahead in life was to rip them off, horribly stunting his social growth. He then carried on this same behavior to his only friends Ed and Double D with the former being too simple minded and the latter not having the fortitude to stop him.
It's only during the movie when this comes to a head and Double D becomes completely fed up with the abuse and resolves to leave Eddy forever. But rather than deflect, Eddy finally looks inward and admits that he was wrong and breaks the cycle, treating his friends like actual friends for the rest of the movie. This again climaxes when we meet Eddy's brother and see just how horrible he truly is. Eddy finally breaks down and admits the truth about his brother and in the process resolves to become a better person which makes him accepted by the other kids.