Just want to add some more points of what you can do on an unused Avios booking, 24 hours before flying:
- You can change the flight time on the same day
- You can change the day of departure
- You can cancel the booking
- You can change the destination to others in the same award band
- You can change the airline that you are going to be on
- You can also add stopovers on the booking (I am not sure about this)
So basically I think you can do anything for a fee and taxes and paying more Avios when needed... am I right?