Wall Street Journal: Ron Howard re-shot 70% of "Solo: A Star Wars Story." Writer Lawrence Kasdan fought with Chris Miller/Phil Lord, and is to blame for their firing.

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The Wall Street Journal has some juicy tidbits in regards to the problems "Solo: A Star Wars Story" had in pre, during and post-production.
Lucasfilm president Kathleen Kennedy is described as person that “too frequently second-guessed her own choices and hasn’t effectively resolved disagreements with directors,” which explains her problems with directors, most notably fired filmmakers Phil Lord and Chris Miller on "Solo" and Collin Trevorrow on "Episode 9" but also kept but "technically fired" Gareth Edwards on "Rogue One."
WSJ also states that Miller and Lord most notably fought with "Star Wars" veteran writer Lawrence Kasdan over the script even before production, with Lord and Miller deciding to rewrite tons of pages from his screenplay. "Retakes and on-set improvisation are commonplace while shooting comedies. However, crew members, who have been around “Star Wars” sets previously started to become “frustrated” and “exhausted” by the directors’ shooting style." Yikes.
Kasdan even “undermined” the filmmakers and even criticized them behind their backs to the cast and crew. Despite all the brouhaha, Lord and Miller did finish the film, but were ultimately fired and replaced by Ron Howard. 
“Ron wanted to go back to the spirit of the original trilogy, while Phil and Chris were looking forward to something new, more like ‘Guardians of the Galaxy.’” The source continues saying that Howard re-shot about 70% of the movie.
On a side note, the WSJ report states that Lucasfilm did approach Rian Johnson to direct the final film in the new trilogy, but “serious discussions” were never developed.