Yeah, it really does depend on the airline and the contract. As long as it does not contradict CAA minimum requirements then it is legal. I once flew for an airline that would not let you leave the hotel before 1000 local time down-route in case of a change. This same airline would also count two of your clear days down-route (we used to have 6 in POP and CUN) as days off towards your 8-in-a-month allocation.
In this case with Silverjet, from an outsiders standpoint, as long as minimum rest is achieved after the phone call, then you would be obliged to operate the change - unless your agreement specifically states otherwise.