Yes you can do this but
- you can use BWC to extract an FTV for the unused sectors prior to September 2022
- historical rather than current pricing applies after you have flown the first sector. So make sure you are in full possession of the accurate facts before pressing the wrong lever. It may be cheaper than you think to change the return.
- research "nested" tickets. Even if the above bullet point doesn't apply, you may be able to find another pair of date which will allow a further visit at a later time.