I think Paul Tweedle already said on this forum how it will work. As described by someone, the player will have an error message when trying to enter a online event using Fury's content.

But since tracks and ships can ben modified after players have joined the event, I don't know if that message will be based on the game version of the host (so even with a Classic event the message will be displayed) or if you need to choose "Allow/Disallow Fury content" when creating an event. Otherwise, people could enter when the event has HD Classic only parameters, and get the message only when the race will start which will be very annoying for those who waited for nothing (I don't care since I will bought Fury >_<).