When you click “Add” in the Change Deck window, that allows you to create a new deck to which to move the cards, instead of moving the cards to an existing deck. Did you get this window?
But it isn’t intended to create new versions of the cards in that newly created deck – it will still just move the existing cards you have selected. If that’s what is happening, that’s expected.
Yes, it creates the new deck and moves the cards to it. Cards can only be in one deck/subdeck, so that’s exactly what you should expect. You don’t want multiple copies of the same card in your collection.
when I use a deck to create a subdeck, really need multiple copies of the same card in different deck, this way, I can keep the source deck as an unmodified source edition.
There are ways to keep an unmodified version of your deck, if you really need to. But can you explain more about why you want that?
Are you trying to keep an unmodified version of the cards – as in, cards that haven’t been studied and are still New? Or trying to keep an unmodified version of the notes – as in, the original material before you started editing fields?