As people have said, Daniel Davis (Georges) was fired from the 2004 production of La Cage. Another Georges has left the role too: in the 2010 revival Jeffrey Tambor replaced Kelsey Grammer after, Tambor stayed with the show for 10 days, give or take, and then withdrew from it due to 'complications from a hip surgery he'd undergone a few weeks prior', though no one bought that.
Emma Hunton left the tour of Wicked out of the blue too, a couple of weeks before it closed. I don't think there was an announcement of her replacement -something Wicked always does-, so there's that too.
Contrary to rumors Betty Buckley was not fired from Into The Woods. She participated in the first workshop pre San Diego and then was in the two week pre Broadway workshop. She did not participate in the show due to negotiation breakdowns.
Paul Muni left At The Grand before it was to come to Broadway. Believe it was over a pay dispute. Show closed in Frisco I believe and was reincarnated as Grand Hotel many years later.
Timothy Nolen - Michael Crawford's replacement in the original Broadway Phantom back a year or so into its run - was allegedly bought out of his contract halfway through - and replaced by Cris Groenendaal (then appearing as Firmin or Andre in the show). Lesser known, opera baritone Nolen was an odd choice for the most sought after tenor part in years (though other baritones have played the role since), reaching for the top notes (as some who have heard recordings would know). I saw him his last weekend in role with Groenendaal watching from the house...The rumor was that Lloyd Webber and Mackintosh were openly heard in a restaurant debating Nolen's casting.
A decade later, Nolen was back with ALW, starring in his ill-fated Whistle Down the Wind, in a baritone' a role. That he was a Hal Prince favorite, in a Prince-directed show- didn't hurt.