A circuit with a specific date confirmed in the calender normally cannot be changed once the final version of the calendar is out except in very unusual circumstances. It’s possible for a circuit to fall off the calendar if it doesn’t pay its dues, doesn’t do the necessary upgrades between races or experiences force majeure. In the latter case, it theoretically can move date but every team must agree, and that hasn’t happened since Spa 1985.
Unless you know of some reason Spain may experience force majuere (and I don’t), it should be safe to go ahead with bookings – I’ve never heard of the organisers of the Spanish GP complaining about the demands the F1 circus makes, so it’s very unlikely that they will neglect the circuit or refuse to pay the agreed amount for the race.
