The NHL doesn't even recognize the KHL as professional hockey, so Demidov could sign an NHL deal even if he had an existing KHL contract, as far as the NHL is concerned. Demidov probably wouldn't do that, because he's not trying to piss off people in Russia for no reason. If he wants to come over after his current deal, presumably he will, and he will not sign a KHL extension. If he does sign an extension, whatever team drafts him will be in the loop, and won't offer him a deal knowing he already intends on playing in the KHL. His current deal expires Apr. 30th, 2025, so unless politics get in the way, I would assume he's coming over after that.