Changed Twitter username not reflected on Keybase

I just changed my Twitter username. The proof-of-ownership tweet is still recognized on the Keybase website and correctly linked to, but the username change is not reflected. Furthermore, if I search for myself@twitter using `keybase id`, or launch the Keybase GUI application, the proof check fails. (You can easily see all of this by looking up this Github account on Keybase.)

Looks like the intended behaviour is indeed for the check to fail, but presently it doesn’t do so consistently. Also, given that _some_ form of persistent account identity seems to be preserved across username changes—the tweet link on the website works—is it really the best idea to just give up? (This depends on which data are stored in the sigchain.)

Reproduce the behavior across the Keybase website, `keybase id`, and the Keybase GUI using an account whose Twitter username changed. Compare the proof-of-ownership result and linked tweet in each entry point, then determine whether the intended outcome is consistent failure or preservation of the account identity.

Written by the indexing model from the issue text.