Replacing someone after work had begun wouldn't be the first time in Hollywood, as Jephael mentioned. There's a lot of examples. Like the role of Aragorn in Lord of the Rings. And I think they replaced the first captain they had for Voyager after they started, didn't they (but before finishing the first episode)? Howard Shore was replaced for the music for King Kong.
I'm sure something like that has to be hard for everyone involved. To start putting your energy into something, then realize it's wrong, or be asked to leave....
Hopefully, we'll find out if it's just a media mistake (and there are certainly, as Mikey pointed out, plenty of those, too!!)