If you use multiple diagrams, all changes made to one diagram are automatically synchronized with the other diagrams as well. In you case, for example, if an aspnet_users class is moved to a diagram different than the default one, all the relations with other classes will remain though their links won't be visible because the other classes are in a different diagram. You should still be able to see the navigational properties and use the "Edit association" context menu command if needed.
Multiple diagrams are just different visualizations of the same model. If a class is added to two or more diagrams, and is edited in the first one, this will affect the shapes for this class on all diagrams. More on the subject can be found in this
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