Hi
Firstly you need to know that in theory you can call yourself any name you like as long as it is not for nefarious purposes. My mother was Julia, always known as Rita.
However, with surname changes like this it could be a number of reasons, just 2 listed
1: falling out with family
2: money - he was asked by a close older family friend to change his name to theirs for a legacy
I had a much older cousin who changed her name and for neither of the above. She was in constant contact with her father. She just probably thought her name was not 'exotic' enough for an Air Stewardess. But following her name change, she then married (under new name so I am assuming her change was legal) and she ended up being Elizabeth Taylor. Sadly not the film star, but she was certainly glamorous enough.