If you review the documentation, as of last night, it was updated to include “or subdomain” to the instructions. The new documentation reads:
Previously, you were unable to communicate to Google in Google Webmaster Tools that you are moving a subdomain to a new domain. Now you can, which is a great addition to the change of address tool. Of course, many still want the change of address tool to support HTTP to HTTPS moves, which is currently does not. In either case, setting up 301 redirects is a really good step to also communicate the domain moves.
Menashe Avramov created a screen shot of the document change, which I will embed below as a PDF:
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