I always check rates for upcoming stays to see if they have dropped, and if they have, I modify the reservation for the lower rate. I have never had a problem with this.

I was holding a couple of reservations at the HGI in Fishkill for the upcoming holiday weekend. When I booked the rooms, it was $200 for the first 2 nights and $130 for the third night. I checked a few days ago and the rates had dropped. I rebooked for $160 and $110 and didn't think anything about it. Today, I went into my reservation and lo and behold, it still had the original prices in it. So I though maybe I was going crazy. I did it again, only this time the rate was $180 and $110. I modified it AGAIN, printed it out. Then went back in my reservation. Old prices ($200/$130).

I called the Help Desk and they told me I'd have to take it up with the hotel. Which I am doing. I think they thought I was making this up. The weird thing is I did the exact same thing with an ES reservation a couple of days ago. I checked it today and it was fine.

But the moral is, the safest path is to completely cancel your old reservation and book a new one. Or if you modify your rate, be SURE you have a copy of your printout.