如何在Amazon S3中更改对象的元数据

If you have already uploaded an object to an Amazon S3 bucket, how do you change the metadata using the API? It is possible to do this in the AWS Management Console, but it is not clear how it could be done programmatically. Specifically, I'm using the boto API in Python and from reading the source it is clear that using key.set_metadata only works If you have already uploaded an object to an Am