I originally booked online, but the winter codes came out a few weeks after I booked.
After I did a rough estimate and realized how much the code was worth to me, I called the CRO. I can state that in my experience, I had no problems getting the code applied to my exisiting web-booked ressie.
I had selected the phone tree option to change an exisitng reservation, and asked the reservationist what my quote would be with the code applied. She returned a quote to me which was about $450 less than what I was paying.
so I asked the big question next "What do I have to do to get that rate?" She responded that since it was a lower rate, she had gone ahead and made the adjustment for me, and sent out a written quotation showing the revised rate. Much better service than I expected when I was essentially calling to get a significant rate reduction.
As always YMMV.