Why did Willy Wonka change his mind?

-At the end of the original Willy Wonka and the Chocolate Factory Carlie walks up to Wanka after being yelled at and gives his his everlasting gob stopper. Wonka mutters something and tells Charlie he won.Because Charlie was the only one who returned the everlasting gobstopper. It was a test between Slugworth and Wonka. Slugworth offered Charlie money for the gobstopper after Charlie won the ticket. So even though Wonka turned him away (it was a test) Charlie still gave the candy back. He didn't want to ruin Wonka for his own profit. The thing Wonka mutters is "So shines a good deed."Charlie could have sold the gobstopper for money once he left, the fact that he gave it back meant that he was a good boy. Or something along those lines.

If you actually read the book almost none of the original movie happened though :)
Willy Wonka changed his mind because Charlie gave back the gob stopper. He knew the other kids would give the gob stopper to Slugworth for the ten grande. Willy could tell Charlie was a generous, polite young man.
Because Charlie had admiration and lacked greed like the other brats in the movie.
willy wonka told charlie he'd lost..to see if he would sell the gobstopper to slugsworth..but charlie gives it back..proving that he is pure of heart
Cause charlie loved him and wanted to work with him... ;D