Helicopter tour companies are flexible when it comes to rescheduling your flight, as its a common thing that is out of the customers hands - and they know that.
So therefore, they will allow you to reschedule your helicopter ride if you need to, providing its out of a 24 hour timeframe in most circumstances.
Therefore, more than 24 hours should give the heli service time to make the rearrangements without coursing rescheduling problems if its less than say 24 hours.
If you need to reschedule your flight under the 24 hour time frame - and that doesn't relate to all services as cut off times vary or there are non at all - it may occur a service charge for inconvenience.