I just went through this, as I just finished my 15k spend. What a disaster! I had an existing reservation with points that I wanted to convert to the reward certificate. I first checked to see if a standard reward room was available (it was) and then called. After being put on hold, I was told it was done but I would have to present my reward # at the hotel to get my points back. I never got a new confirmation and the certificate number on the reservation did not change. I was not real comfortable that this was correct, so I cancelled the reservation online and called back the next day to rebook with the certificate.

When I called back, I was told I had two certificates. One which expired next September and the one I just got. I told them to go ahead and use the September one for the reservation. Only after making the reservation and hanging up did I realize the expiration date was exactly one year from when I made the previous reservation and the new reservation did not have the certificate number of the free weekend night reward. I also never got my points back. I realized this was most likely also incorrect, so I cancelled this reservation and waited for the points to reappear in my account. In about a half hour, the points were back and I called again. Everything seemed to go ok this time as the reservation has the same certificate number as the reward and no points were deducted.