I may be splitting hairs here, but the Change of Address form on WMT states "new" domain:
If you've moved your site to a new domain, you can use the Change of address tool to tell Google about your new URL. We'll update our index to reflect your new URL. Changes will stay in effect for 180 days, by which time we'll have crawled and indexed the pages at your new URL.
If I am moving a site (Site A) to a domain I already own (Site B) that has been active for a long time, should I use the Change of Address form?
I've done the 301 redirects, and that all looks clean, but if Site B has a much broader scope than Site A, is the Change of Address for appropriate? Site A is effectively a subset of Site B.
Edit: This great webmaster tools video explains this pretty well, but my scenario is slightly different in that a domain is effectively "absorbing" another domain. In the video all three domains end up somewhere new.