You did not need to cancel the flight and rebook. You simply click on Change Reservation and select the new flights. That way you don't run the risk of the flight being full from someone else booking at the same time. Sounds like everything went fine anyways for you.
The credit is good for 1 year from date of original booking, not the date of your flight. if you fail to use the credit by that date you will lose the credit. After Jan. 28th 2011 the credit can only be applied towards a flight for the same passenger as originally booked. Until then the credit can be used by anyone you choose.