For every other member of the Royal Family, the death means they jump up one place in the line of succession, and the biggest changes will come for William and his wife Kate as they have now become the Prince and Princess of Wales.
But it appears that two members of the royal family who won't have a change of title are the Duke and Duchess of Sussex's children, Archie and Lilibet.
Harry and Meghan stepped back from royal duties in March 2020, and did not give royal titles to either of their children, though they could have opted to use a version of one of Harry's titles, the Earl of Dumbarton.
Upon Charles - their grandfather - becoming King, they are now entitled to be a prince and princess.
But Buckingham Palace said the duke and duchess made a personal decision that he should be plain Master Archie Mountbatten-Windsor instead, and Lillibet will remain Miss Lilibet Mountbatten-Windsor.
A royal source said following Archie’s arrival that the couple had chosen not to give him a courtesy title “at this time”.
The Sussexes stopped using their own HRH styles after stepping down as senior working royals for a life in Los Angeles, and there were later calls for their duke and duchess titles to be removed.
If Harry and Meghan decide that Archie and Lili will be known as a prince and a princess, this will stand in direct contrast to how Harry has always spoken about the burden of having a title.
In an interview with Oprah Winfrey in March 2021, Meghan suggested that it was not her call that led to her son Archie not having a title bestowed upon him when he was born.
Meghan indicated that the decision not to make Archie a prince was made by the palace.
In 2021, the Duchess of Sussex told Winfrey: "They didn't want him to be a prince or princess, not knowing what the gender would be, which would be different from protocol, and that he wasn't going to receive security.
"In those months when I was pregnant, all around this same time, so we have in tandem the conversation of, you won't be given security, not gonna be given a title and also concerns and conversations about how dark his skin might be when he's born."
But Archie was born too far down the line of succession to automatically become a prince or use HRH, due to rules set out more than 100 years ago by King George V.
The title of prince and princess is automatically bestowed on the grandchildren of the monarch – which is why Beatrice and Eugenie became princesses when born.
Prince Edward and Sophie decided not to use the HRH titles for their children, but Louise and James could decide to be princess and prince when they turn 18.
Around the time of the Winfrey interview, ITV's royal correspondent Chris Ship explained: "The grandchild of the monarch automatically get HRH, prince, princess.
"Meghan is alleging that Buckingham Palace were trying to change the rules so that Archie would not become a prince."
The Queen had already changed rules about titles for William and Kate's children, a move made before Harry was dating Meghan and making plans for their own family.
The change came at about the same time as the end to male primogeniture, meaning no younger brother of the Duke and Duchess of Cornwall and Cambridge's girl would be above her in the line of succession.
It's why Charlotte maintains her spot despite having a younger brother – but it doesn't apply retrospectively, so Princess Anne is below both of her brothers.
The Queen also made the decision then to give her Cambridge children the right to use the titles of prince and princess, even though they would not have been able to until she died under the previous papers.
Much has been made for many years about Charles's plans to slim down the monarchy before he become king, wanting to have fewer titles and fewer working royals carrying out engagements.
But it's also been suggested that pushing Harry out was not in his plan before the Duke and Duchess of Sussex opted to leave their roles and move to the US.
Charles will have a series of formalities and engagements to attend to before he starts thinking seriously about what his reign looks like.