I would very strongly advise against doing this in this order. Make the new reservation first, then cancel the old one. I've done it the other way with two screens, cancel, the one I had, and when I clicked to confirm the new rate, lo and behold, it wasn't available, and the rate they offered was higher then what I had.

Always make the new one first, then cancel the old.

One other bit of advice, make sure you check rates logeed in and not logged in there are times the rates are significantly lower if you are not logged in, you can login after you have selected the rate.