Change subdomain to addon domain?

I know I've done this before on other hosts, but can't seem to get it to work - any help?

I set up a subdomain to work on a client site:

Now the site is ready to roll, and I want to assign as an addon domain that points to

But, I am getting an error:
Error from domain wrapper: Domain already exists, it was not added.:

Any ideas?
If you don't feel comfortable editing the config directly, copy the files, delete the subdomain, create the addon domain, then copy the files back to their new home. Should work.