- Upgrade an old SHA1 to SHA256.
- Replace an old manually configurable HMAC key with an automatically generated one.
This is generally both simpler (less configuration) and more secure (you now get a unique value automatically).
This causes a one-time compatibility break that invalidates old "Reply-To" addresses. I'll note this in the changelog.
If you leaked a bunch of addresses, you could force a change here by mucking around with phabricator_auth.auth_hmackey, but AFAIK no one has ever used this value to react to any sort of security issue.
(I'll note the possibility that we might want to provide/document this "manually force HMAC keys to regenerate" stuff some day in T6994.)