Cant change passenger details

Good day,<br> <br> I made a booking and my husband made another booking for the same day for me and my daughter. So now I had to change my Kulula booking for a later date but now on that date I will only be travelling alone not with my daughter and according to the call center agent I cannot make a passenger change or use her ticket for my returning flight? What will happen now as I will be travelling alone on that date? will that seat be given to someone else but I paid for it?<br> <br>