Why would that possibly threaten the construction set? The CS is what they use to make the game. If they use a new engine, they'll have a new CS along with it, same as it was going from Morrowind to Oblivion.
Well, if they licensed a new engine, rather than building their own, there might be copyright issues if they were to release a construction set; at least I think that's the case.
Of course, we don't know anything about the new engine yet, so these threads might be a little premature.