This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
As Ben said, there is no time limit on the assignments, so you can continue on the FBNS assignment with no problems.
If you want to, you can try downloading the assignment again. You might get one of the new assignments. Then it will be entirely your choice whether to continue with the old assignment or start using the new assignment. Personally I don't see any value in swapping assignments mid stream though.