Does anyone know how to change the ACL of an existing (already uploaded) key?

It's a terabyte of files I would prefer not to have to upload again, but don't see how to just PUT the x-amz-acl:public-read header onto a file.

It seems like I should be able to do:
./s3cmd.rb put bucket:key x-amz-acl:public-read
... but that, of course, is missing the local-file parameter.

Any advice appreciated.   Thanks!