You have made a lot of assumptions in your post, but it isn't clear whether your original instructor changed the appointment, or your new one did, or whether DVSA had to move the date for operational reasons.
To find out what really happened, call 0300 200 1122 (before noon on a weekday) to establish whether the change was made by DVSA or online. If the change was made online, that narrows it down to two people.