Continuity and Oz don't seem to go together well. There's various discrepancies in the Famous Forty, even within Baum's founding fourteen. Still, with a few glances away from certain spots, they largely still work together as a whole.
However, in a lot of recent Oz works, there's the heavy trend of divergent continuity. Things just don't match up with the Baum books, from The Laughing Dragon of Oz to Wicked (both the book and musical) to almost everything done now. From the looks of it, despite it being based on the book, Oz the Great and Powerful will be yet another divergent continuity. It'll be the polar opposite of Disney's Return to Oz: whereas Return was a sequel to a Wizard of Oz we didn't see, it seems Oz will be a prequel to a Wizard of Oz we haven't seen. Or have yet to see. (And people expect both to fit with the MGM film. Sorry, it's not going to happen.)
Some fans aren't all right with this. And I can see where they're coming from. Despite all the continuity gaffes, it's actually not that difficult to set a relatively continuity-friendly story in Baum's Oz.
On the other hand, I've gotten to the place where I tend to accept it. In the case of Oz the Great and Powerful, it seems more like they're beginning a new series, opting to start before The Wonderful Wizard of Oz. And sometimes, trying to strictly follow pre-established continuity can limit your creativity. And it's a movie, a different medium than the original Oz books. We let Peter Jackson get away with Elves at Helm's Deep and Frodo going to Osgiliath in The Lord of the Rings (and The Hobbit is much in the same vein). A film adaptation needs to breathe on its own.
Still, it's a little annoying, because you've spent so much time with the characters in Baum's series that you want to know about their further adventures, and divergent continuities don't offer you those characters, but characters based on them. This might be a fun way of looking at these characters, or you might feel like shouting "Impostor!"
Anyway, in the end, it's Oz, and Oz is supposed to be fun. So, let's have fun with it, shall we?