I know that all. But this only modifies current opened notebook.
Suppose you have two notebooks in files book1.nb and book2.nb.
You have one opened window with book1.nb in it.
The task is to open book2.nb in the same window were book1.nb is.
If you use NotebookWrite[EvaluationNotebook[], Import["book2.nb"]] in book1.nb you just modify book1.nb with data of book2.nb.
Opening in the same window should work like it is with documentation (help). When you have help file of Plot function and click on Plot3D the help file of Plot3D is opened in the same window.
Now do the same with book1.nb and book2.nb like it is with help file of Plot and Plot3D.
And of course it must be done without any resaving of notebook.