What happens if a commercial plane makes a long flight on the last day of the current AIRAC cylcle, ..........

Question

... but has not landed after the new AIRAC is active?

(I mean, let's imagine a flight from Los Angeles to London, departure on July 31st in the afternoon... And it is the last valid day of the current AIRAC0807. Shortly before arriving in the morning at London, the new AIRAC0808 is active (1st day), however, the FMC works with the old one.... So what do they do? Can they load the new one while in flight, or is it not that terrible to fly some hours with the old one?

Answer 1

The FMC has 2 databases.  It always has the current one and the next one stored in its memory.  If it becomes necessary during the flight to switch, it is an easy task to simply tell it to use the other one.

Changes to the FMS are published well in advance of becoming current.  This allows maintenance personnel to load the upcoming database into the FMC's memory up to 2 weeks prior to it becoming active.