hi there...
CAMU have just receintly introduced DEP and ARR slots to all the international airports here in SA. if a flt is from one international to another and the pilot calls early we are not allowed to approve start until it is within that particular slot due to the destination working on arrival slots.
if the pilot wants to leave early then a new FPL has to be filed in order to receive a new DEP and ARR slot.
the only other way out is to cancel IFR and go VFR,which is a no go zone for airliners.