If I reset my pc with the windows 10 (keeping the files) will it reinstall without problems?

If I reset my pc with the windows 10 (keeping the files or remove everything) will it reinstall the windows 10 without problems, for example: will it ask me a activation key (product key) because the windows 10 now is not free? Or if it asks me, can I use the activation key that I used when I bought this pc with the windows 8.1?

I just don't want to make a mistake and lose the windows 10

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