Its never to late to start over. I have started over dozens of times. The assignment counts as being done even though it was done for the story you now hate. For your own personal progress, I would redo them but you can just pick up with the assignments now and keep going.

Its better to start over now than it is to get 10k into your story in November and realize you hate it and its going no where. I've done that too. It's a lot harder to start over even though the words you wrote still count in November you end up feeling like you didn't really do the challenge if your story isn't finished and you only have 40k on the story you changed to and the 10 k on the one you abandoned. Its how I joined the over achievers club the first year. I started like four different stories before I got the one I wanted and it pushed me to hit 50k with the one I ended up doing.