Early next week is when the switch will be turned, to point news.mozilla.org to the Giganews server; but the actual newsgroup transition will happen after that.
Because Giganews contains usenet groups, it already has the netscape.public.mozilla.* hierarchy. When the switch happens, those groups will be available to people accessing the Giganews server via the servername news.mozilla.org. In fact Giganews has had the new groups up and ready since the beginning of October. They’re just not open for access.
After the switch has been made, each new mailing list will be created. There will be an announcement on each mailing list/newsgroup warning subscribers approximately a week in advance of the switch.
Let’s say, for example, you are on the mozilla-qa-general@mozilla.org mailing list, which is bi-directionally mirrored with the newsgroup netscape.public.mozilla.qa.general. When the mozilla-dev-quality@lists.mozilla.org mailing list is created, you will be automatically included on the subscriber list, and Giganews will open the group mozilla.dev.quality for access, at which point subscribers to the netscape.public.mozilla.qa.general newsgroup will have to subscribe manually to mozilla.dev.quality.
If you’d like to see which lists have already been created, you can keep track by visiting https://lists.mozilla.org/.
Right now, netscape.public.mozilla.webtools is the ‘guinea pig’, which will be split into four separate groups. January 4th is the date set for the creation of the mailing lists dev-apps-bugzilla@lists.mozilla.org, dev-apps-webtools@lists.mozilla.org, support-bugzilla@lists.mozilla.org, and support-webtools@lists.mozilla.org, at which point the newsgroups mozilla.dev.apps.bugzilla, mozilla.dev.apps.webtools, mozilla.support.bugzilla, and mozilla.support.webtools will be open for access.
If all goes well with the ‘guinea pig’, the list of available groups will increase shortly thereafter. I’ve been told that after the webtools list is transferred, the rest will happen the following week; but lets all assume a problem or two will arise, and put a buffer week in there. 
