Earl of Jersey added Child to the family surname in compliance with the will of Robert Child who was grandfather of Lady Jersey. Even Byron added Noel to his surname. He and Lord Holland incorrectly added names to their titles as well, probably in an excess of caution as to fulfilling the terms of some will.
Fanny Burney wrote a novel based on a legacy to a girl of a tidy fortune when she married if her husband would change his name to that of the benefactor. The man who said he loved her refused to change his name so left her prey to all sorts of problems.
Most people were willing to change or add a surname if the change came with money.
In Jane Austen's family there are the Austen Knights and Austen Leighs. Byron added Noel to Byron and Jersey added Child to Villiers. Some families had four surnames. Some families did change surname to appear more aristocratic or sophisticated. A Davy Jones , tired of jokes about his lover or the ocean, changed his name to David St.Paul
One man-- a Thomas J Jones had his son's name changed from Vere Jones to Vere Jones Vere. Deadman changed to Dedman. The name given a child at baptism wasn't supposed to be changed except that the Bishop could do so at confirmation if he felt the name was inappropriate. Some thought that a person could only marry by a baptismal or legally changed name. The courts treated each case separately.
However, people did change first names. The name change was only invalid if done for fraud such as pretending to be someone else or to have a clandestine marriage even with banns being called.
The informal way to change a name was just to tell people the new name and insist upong being called that. If one had lived for years in a village under one name then marrying under the baptismal name, which no one knew, could invalidate the marriage.
One could change one's name for any reason or no reason. The name couldn't be blasphemous or profane , nor could one change one's name to The Prince Regent or Princess Sophia.
Joshua Bug liked the Duke of Norfolk's name so changed his to Norfolk Howard.
One obtained a royal license to change one's name by making a application through the Herald's office.
It had to be drawn up with care so as to achieve the exact name requested.
The Royal license is given under the Sign Manual and privy Seal and is countersigned by the Secretary of State for the Home department.
The cost was 10£ when changed for one's own reason's but the stamp tax paid for a change according to a will was , as I mentioned, 50£.
Those fees were the stamp duty. Other fees were charged.
£ 34 to have the name recorded by the College of heralds.
£ 10 to the Exchequer
£ 2 2s to advertise the name change in the Gazette
66£ additional if a coat of arms is issued at the time.
Research by any of the heralds was an extra charge as was research into antecedents for coats of arms. So, a person needing to change a name to receive a legacy had to pay around £100 if he already had a coat of arms.