If contest ends and goes into judging mode, you can still change the ranks as you see fit and when its in judging mode, only the ranked #1 entry / designer can do modifications and submit them, then if you want another designer to submit you some changes you requested, you'd move that other designer and rank his entry as #1 so he can submit during the judging mode.
Only when you are sure about your winning logo, you will mark contest complete with the entry you chose as your winning in #1 and then award other ranks based on how good or close they were to you.
Hope that answers your question.
Do the rankings now all you want, based on the ones you like the most or feel closest to your own preference / vision.
And when contest ends, it goes automatically to " judging " mode where you still can re-rank and change orders the way you see fit.