He was a notorious gangster. After his marriage, he dissociated himself from the gang. This sudden transforming / transformation of identity surprised all his close friends and the police.

Can either bold word be used? And does "of identity" fit in the sentence?

Thanks..

Could someone please help? Thanks.

I'd go with transformation. Transforming would refer to the process of changing his identity, transformation to the result.

Faldage is right. And "of identity" is tautological---excess baggage, already implied.

Near the end of that sentence is a rare instance when "as well as" might work better than "and." It emphasis the object and sets up the punchline.

I'd leave "of identity" in. It helps clarify just what the transformation was.