Name Change of an Individual in New York City, Early 20th Century

My great-uncle Angelo arrived at Ellis Island in 1898 from Italy. Up until around the time of his marriage in 1908, his given name was listed as Angelo. Angelo was also the name on his vital records in Italy. When he filed legal papers to enter a business partnership in a plumbing business in New York City, his name was listed as Charles. From then on, all documents (naturalization, draft registration, letters of administration for his estate) list his name as Charles. I am positive that Angelo and Charles are one and the same person from first-hand accounts from family members who knew him by both names.

Here is my question: Was it possible at that time in New York City to change a given name without going through a legal process? If a legal process was required, where would I find those records?