I did that -- I had an existing ressie for AKL in August '08, and when I was at CSR last weekend I re-booked the same date and room type using the bounceback offer code. Saved me $200!! (on a 2-night ressie).
It may depend whether they still have what you want, because they cancel out the old ressie and book a new one. At least that's what they did with mine. The first CM told me that there was no availability for my dates using the bounceback code, and could not apply the code to my existing ressie. I called back and got another CM who took care of it.