What does a preliminary cast mean in a movie?

[Football Guy]

Two year before a movie comes out, the cast is called preliminary. Does that mean the cast can be changed? Or they are doing casting calls? please give details about what it is

 

[RunningBear]

It means the people cast have said they'll be in it, but they're not firmly committed yet because it's so far away, so things might change.

 

[Jeff]

It means that they're somewhere after the main casting, but not filming yet, so a change COULD happen.