Harry saves Draco from being mauled by Buckbeak, which means he first match of third year is still Gryffindor-Slytherin. The dementors show up like in canon, Harry falls—and Draco catches him as repayment, meaning nobody won and the match will be rescheduled.
Harry then gets Flint's backing to move it to the last match of the year anyway, since their houses are rivals and they need sufficient dramatic build-up.