Document URL update procedures

## The Problem

1. User logs in to example.com with username.blogplatform.com/profile as their ID.
2. User repeats step 1 for several clients.
3. User changes their ID to fullname.io
4. This user is smart and sets a 301 redirect from username.blogplatform.com/profile to fullname.io
5. User logs in to example.com as fullname.io
6. User now sees they have created a new account, rather than keeping their existing account on example.com that they may expect. The user could log in using their old URL, but as far as the site is concerned, the user is only logging in as fullname.io

There is also the risk (though likely a small one for savvy early adopters) that a user loses control of their domain. Perhaps they let it expire and the DNS records now point elsewhere.

The issue mentions no documentation file, test, or entry point. Start by locating the existing guidance for URL-based identities and account changes; done means documenting the update procedure, including preserving account recognition and the risk of losing control of an old domain.

Written by the indexing model from the issue text.