See parent task:

As part of T348388: Use central login wiki for login (SUL3), the login and signup forms would be moved to the central login wiki. It would make sense to move the credentials change forms (password change, 2FA change) as well. This doesn't block rollout of the other SUL3 work, and could be done much later, apart from WebAuthn where we'd have to disable the local form at least (T354701: Enable migration of WebAuthn credentials to loginwiki).

In the simplest form, this could just mean changing the various Special:Preferences buttons to be simple links.

Once we only use loginwiki to do password or 2FA change, existing accounts not attached to CentralAuth will break. Proposed:

• For each wiki, enumerate its user table (we do not use localnames table, since local user table is canonical) and compare it with globaluser table.
• If the local and global username differ, unattach it first since CentralAuth can not merge accounts with different names. I am not sure whether there are any such cases.
• If there are no global account with a specific user name:
  • If local account with such name only exists in one wiki, create a global account for it.
  • If local account with such name exists in multiple wikis, we need to first rename the account so they have different name, then create new global accounts.
• If there is a global account with such name:
  • If local account has zero edits, it can be attached to the global one.
  • If local account is a system user, it can be attached.
  • In other cases, they should be checked manually before attaching.

See also T367968#9908254: this is basically doing SUL finalisation again, but in a much smaller scale.