I could not find a bug/issue to cover this work, but per Dustin in email:

On Mon, Nov 4, 2019 at 12:40 PM Dustin Mitchell <dustin@mozilla.com> wrote:

Login *should* work (we'll update it with new creds), but it's untested. 
So ideally we'd at least have this ready to land on Monday  as a break-fix
if login.taskcluster.net doesn't work.  There will still need to be some 
changes to point at the new rootUrl.```

This doesn't strictly block the switchover due to the workaround, but this work should happen ASAP.

Done in https://github.com/mozilla-releng/treestatus/pull/1