But what was the reason for the shift from Charlie to Willy Wonka in the film title?
From the Dutch wikipedia about that movie:
De titel van het verhaal werd echter aangepast naar Willy Wonka and the Chocolate Factory om beter aan te sluiten bij het snoepgoed dat in de film gepromoot zou worden, en omdat in de Verenigde Staten (die toen betrokken waren bij de Vietnamoorlog) Charlie een spottende bijnaam was voor de Viet Cong.
The title of the story was adapted to Willy Wonka and the Chocolate Factory to better connect to the candy promoted during the movie and because in the United States (who were involved in the Vietnam war at the time) Charlie was a derogatory nickname for the Viet Cong.
Screen Rant suggests there are 4 reasons that have floated around over the years:
One is that the specter of racism reared its head again — apparently one of the films producers, stung over the NAACP’s accusations over the Oompa Loompa’s, noted that slaves called their bosses “Mr. Charlie” during the war. Another theory is yet another racial implication: that the word ‘Charlie’ was a derisive term for the Viet Cong by American Soldiers during the Vietnam War.
The other theory is less culturally sensitive and more corporate: since Quaker Oats was using the film to market their Wonka Bar, they wanted a title that best reflected the brand. Whatever the reason, Stuart made the most compelling argument for the name change, saying “If people say, ‘I saw Willy Wonka,’ people would know what they were talking about. If they say, ‘I saw Charlie,’ it doesn’t mean anything.”
All four reasons are also mentioned in this article
Is it not more of Charlie's journey in the film? Is it for the hint to any supposed deviation from novel to film in the film?
Yes, Roald Dahl was disappointed about the movie, and the focus placed on Willy Wonka instead of Charlie's development 1
But none of the sources I found suggests that the change of title was made as a premeditated hint to any supposed deviation from novel to film.