Strange as it may seem, you can’t directly rename folders in the Amazon S3 online console:
Files yes, but directories no. As I had a batch to do, this was a bit of a pain.
There do seem to be wee Windows S3 clients that will offer this functionality though. One which came to the rescue was the free S3 Browser which did the job, though just one directory at a time. Install, plug in your access credentials and away you go.