The short answer is that no, the assigned versions will not change.
Think of the version in your library like the master copy, and assigning it to a class is like running off copies on the copy machine. If you change the master copy after you run off copies, it does not change the version that the students have.
Although there are cases where it would be convenient to update all of the assignments at once, it would cause confusion in many other scenarios. For instance, what if a teacher edits a shared assignment? Or if you edit an assignment that you assigned four weeks ago - should it still update those assignments?
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