I misunderstood the following: "An existing booking can be changed once, depending on the availability, but only within 60 minutes of the scheduled start time." The scheduled time was 9:30 AM and the changes were accepted no later than 8:30 AM (or before). My understanding was that the booking can be changed between 8:30 and 9:30 AM, so within 60 minutes (before the start).
Any help/feedback would be much appreciated!