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Lewis did begin a early version of Magician's Nephew (The Lefay Fragment) just about the same time he was finishing up LWW. The final version of the book was quite different from the fragment, but Digory and Polly were both in it. It's quite possible he had planned on making Digory a king in Narnia in the first try at MN. Of course, that does also assume that he planned on having The Professor of LWW be Digory, something that's not stated in LWW.
Selah,
Sven