I am not mod so do not take my advice as authoritative but I do agree that it is a grey zone. Someone could have submitted a game made before if it somehow fits the theme nicely or instruct your whole game studio to work for the game. I think the intention is to encourage the creator to create as many new things as possible. If you feel like yourself simply swapping out something/re-skinning from the old game then probably it is not ideal for the jam.
My personal take is that do not see this jam as a competition and the winner should not be incentified. Try to always create a game that has some new elements in it whether it is new genre, new mechanism, new set of assets. In your case I would say just have a new gameplay mechanism is enough to justify even you reuse all the assets.