I have a reservation thru CRO that I have modified over 5 times, so hopefully I can be of some help!
Each time I modified my ressie was to add on extra days, or change the check in day. Changes like these require the CM to check availablity on those days I wanted to add on. While the reservation stays the same, meaning it's not a new ressie since you're just modifiying, more than likely you will be given a new reservation #.
All the times I called to modify my current ressie, I was given a new reservation # each time. And each time they sent out a new reservation showing the changes.
Everyone seems to have different experiences with CRO regarding the same situation, but this is what I experienced. I do agree that you should call CRO.
Diznee25