Conversion is a change of attitude, emotion, or viewpoint from one of indifference, disbelief, or antagonism to one of acceptance, faith, or enthusiastic support, especially such a change in a persons religion.
An example in A Separate Peace is when Gene thinks that finny is planning for him to do bad in school so he can catch up, making Gene completely change his attitude and views towards Finny into a more negative way of thinking, which later causes his him to hurt Finny.
"Conversion." Dictionary.com. Dictionary.com, n.d. Web. 02 Feb. 2017.