You really should be able to (for a fee) be able to "unsign" from Dusk or Dawn as long as its in the signup period. This sunday I was signing up and without thinking went and signed up for Dusk and not Dawn, ruining not only my own leveling spots, but for my friend too and wasted my friends signup money :(
He wasn't borrowing his friend's money to sign up - it sounds to me like he is in a dedicated duo, and that now he won't be able to train in his usual spot, and thus his friend, who signed up to Dawn correctly, can not either.