[flamingGalah!]

It would be a contract change as they would have signed to stay in there potentially as long as the series was due to run. If Channel 5 decide to extend it, then they need the housemates to agree, as some could have commitments etc.. after Big Brother... The leaving before it ends is a bit different to them staying longer than they were ever contracted (& paid) to, so they would have had to negotiate with the housemates & their agents first...