All your ticket guarantees is passage from point A to point B on the given day. If the airline changes your flight time substantially (widely disparate definitions of this word depending on the airline), they may refund your fare - but then you would need to purchase a ticket on another airline at a much higher price.
Mind if I ask why you're asking? Does your airline have a history of cancelling a flight on which you're already booked? Are you thinking of buying an inexpensive ticket in the hope that the airline will cancel that flight and put you on a more desirable but more expensive one? Something else?