As you said, there are no rules regarding how far the cast moves. A smart creative team is looking at casting with an eye to the future, but there are so many variables that come into play. Somebody may get cast in something else, schedules could change, the creative team could change, something could happen in the workshop that completely alters the direction the piece is going in, characters get added, characters get cut, and the list goes on.
There are rules about the type and number of auditions that must be held at each stage. Call backs are not required by the union and do cost money so there's little point in holding them if the cast from the workshop is moving forward to the next stage (whether that be another workshop, out of town production, pre-Broadway tour, etc).
There's been more and more of a push within the industry to get more transparency regarding audition notices in respect to what is going on with casting, such as noting whether a role is already cast, if they are specifically seeking a "star," if an offer is out but not yet accepted, and other such items.
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