Victor Fleming replacing George Cukor on GONE WITH THE WIND
It wouldn't hurt to read the linked article, R2.
Victor Fleming replacing Cukor on WIZARD OF OZ.
Joseph Mankiewicz replacing Rouben Mamoulian on CLEOPATRA
Otto Preminger replacing Rouben Mamoulian on LAURA
Stanley Kubrick replaced Anthony Mann on Spartacus
Richard Lester replaced Richard Donner on Superman II
James Stewart and Anthony Mann had a tremendously successful relationship; their westerns are under-rated gems. However their 6th western NIGHT PASSAGE turned into a debacle when James Neilson replaced Anthony as the film's director. The film flopped and James Stewart never spoke to Anthony Mann again.
Norman Jewison replaced Peckinpah on [italic]the Cincinnati Kid[/italic].
[quote]Victor Fleming replacing Cukor on WIZARD OF OZ.
Cukor replaced Richard Thorpe, but he was getting ready to start GWTW, so it was always meant to be triage and just to get the production on track while they found a permanent replacement. So, technically, Fleming replaced Thorpe.