That just happened to me, Michelle, when I booked with National, here's what I did:
I booked a year in advance and got a pretty decent rate.
When the new coupon came out, I tried to call back and re-book as advised above, but I was told I'd be paying MORE than I would be if I kept my original rate!
You see, when they re-book they have to use today's rates and take the coupon off from that.
Now, if you book on-line you will do better, I used their 800#. Good luck.