My understanding:
First leg cancels and you are on the hook for reassignment up until original report time +6 hours (no change to current contract)
Any reassignment during that 6 hour window will come with no less than 12 hours to the new report time (as opposed to "promptly available")
So, there is no obligation of the company to do anything to your cancelled first day until as late as original report time +6 hours and they must have you back within your original footprint +4 hours.