adding new students to a class in progress

Yesterday I added a new student to a class that was already in progress.  She was joining a week after the other students and there was a week's worth of past assignments that I needed to assign to here.  The only way I could accomplish this was to change to due date on the assignments so that they were no longer in the past. 

How does this affect the students that have already completed the assignment?

Also I had carefully picked the time of day for assignments that were due on the same day so that I could be sure that they appeared in the correct sequential order.  When I had to assign them to her I forgot that I would need to change those times because now they were all being assigned on the same day.  This created a jumble of assignments all due on the same day out of order.

Was there a more sensible way to get all of the assignment to her without ruining all of my previous work?