There are any number of circuits who could step up though - if Bernie named an acceptable price.
Bernie can't readily change the date - Valencia is two weeks after Montreal, and two weeks before Silverstone (likewise, moving the race to August means it will be two weeks after Hungary and two weeks before Spa). Any change to the date will have a serious knock-on effect for other events, affecting circuits, teams and spectators. If the race is to take place, then it's going to have to take place on June 24 or August 12. They are the only two dates that can be managed.
Because of the events before and after both dates, the race will not be a flyaway. The teams would never agree to it - it would jsut be too expensive, and too difficult to turn things around in time for the next race. So the race has to remain on the European continent. There are plenty of circuits in Europe, but any replacement venue would, at the very least, need a Grade-1T licence from the FIA. There are only a handful of circuits for which this is possible:
1)
Portimao in Portugal. It has a 1T licence, and the idea of a race taking place there has been popular since it was built. But Portugal is one of the PIIGS, with a very delicate economy right now. Unless private funds could be found for the race, it's probably not going to be feasible, even with the discount.
2)
Motorland Aragon in Spain. It's massively popular as an event, with many people wondering why Formula 1 goes to Barcelona and Valencia when Aragon is available. But the circuit is in Spain - another of the PIIGS - and only has a Grade-2 licence. Bernie approached them with the idea of filling in for Korea if Korea was not ready in 2010, but the circuit said no.
3)
Imola in Italy. The event might have been run as the "San Marino Grand Prix", but the circuit is in Italy, which like Portugal and Spain is one of the PIIGS. Even though the final chicane has been removed, and the circuit has a 1T licence, is simply too small and too cramped for Formula 1.
4)
Red Bull Ring in Austria. Unlike the other three, Austria is not one of the PIIGS. It's backed by Red Bull, so they could certainly afford it; doubly so at a discounted rate. But, like Imola, the circuit is too small - the lap record is a paltry 1:08.337, and that could easily be beaten. Furthermore, Dietrich Mateschitz has said that Moto GP and Formula 1 will not visit the circuit while Red Bull runs it.
5)
Magny-Cours in France. The French want in, Magny-Cours has the appropriate licence, the economy is stable, and the circuit's notorious traffic issues have been resolved as a highway now goes straight past it. But the French are believed to be favouring Paul Ricard for 2013 and beyond, and while the owners of Magny-Cours want the race for themselves, a lot will come down to who the French government is willing to back.
6)
Paul Ricard, also in France. Believed to be France's favourite venue for 2013. It has a 1T licence, but the major problem is that the circuit has no spectator facilities.
If Bernie decides that Valencia aren't worth the trouble they cause, and wants to hold the race elsewhere, then I think Magny-Cours is the most-acceptable venue, though it may create controversy if Paul Ricard is the favoured circuit. But I still think that, because of the time constraints and the need for a discount to make any replacement race viable, Bernie will tolerate Valencia for one more year, then kick them to the kerb and replace them with New Jersey. It's the easiest solution.