It's messy, but Nine did hold the rights for the first two years it was off air, which they paid for (not full rates, and the agreement accounted for that). Will try and explain later when I have some time.I wonder though if like Nine whether Seven will have a lengthy opt period should they axe the show. It was ridiculous that Nine seemed to have the rights to prevent anyone else showing Big Brother for a longer period than they actually showed Big Brother (5 years vs 3 years), with Seven pretty much snapping it up as soon as Nine's option on the format expired.
No-one could prevent Endemol from selling the show to another party if the rights aren't being actively held by someone. Everyone tries to bake resting periods into these contracts, but unless it's paid for, it's third line forcing.