Provide a way to indicate that a user's account has migrated to a chosen username.

### Your use case

#### What would you like to do?

I want to be able to indicate to other users that I have transferred a deprecated account to a replacement account, in a manner that is verifiable.

#### Why would you like to do it?

In lieu of [`matrix-spec-proposals/pull/1228`](https://github.com/matrix-org/matrix-spec-proposals/pull/1228#issue-322001991) being ready (and/or what [`matrix-spec/issues/220`](https://github.com/matrix-org/matrix-spec/issues/220#issuecomment-5553301233) describes), I have accrued multiple Matrix accounts, because I have wanted to replace my username multiple times:

- 1. [x] `matrix:u/@rokebeedell:matrix.org`

1. [x] `matrix:u/@beedellrokejulianlockhart:matrix.org`

1. [x] `matrix:u/@rokejulianlockhart:matrix.org`

1. [x] `matrix:u/@beedell.roke_julian_lockhart:matrix.org`

1. [x] `matrix:u/@beedell.roke:matrix.org`

- 1. [x] `@beedellrokejulianlockhart_gitlab-62fbf1f66da03739849b6cae:gitter.im`

1. [x] `matrix:u/@rokejulianlockhart:gitter.im`

1. [ ] `matrix:u/@beedell.roke:gitter.im`

- 1. [x] `matrix:u/@rokejulianlockhart:beeper.com`

1. [ ] `matrix:u/@beedell.roke:beeper.com`

[^1]

[^1]: [`reddit.com/comments/1ea7b9b/comment/p802rvb`](https://www.reddit.com/r/beeper/comments/1ea7b9b/comment/p802rvb/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button)

- 1. [x] `matrix:u/@beedellrokejulianlockhart:fedora.im`

1. [x] `matrix:u/@rokejulianlockhart:fedora.im`

1. [ ] `matrix:u/@rokejulianlockhart:fedora.im`

- 1. [x] `matrix:u/@rokejulianlockhart:mozilla.org`

1. [ ] `matrix:u/@beedell.roke:mozilla.org`

- `matrix:u/@RokeJulianLockhart:chat.suyu.dev` [^2]

[^2]: [`reddit.com/comments/1c10p18/comment/p7y61dh`](https://www.reddit.com/r/suyu/comments/1c10p18/comment/p7y61dh/?context=3&utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button)

Although [`discussions/1832#discussioncomment-14684837`](https://github.com/element-hq/element-meta/discussions/1832#discussioncomment-14684837) mitigates the effect of that, I want those who want to contact me to be able to quickly and verifiably be informed that they should contact `matrix:u/@beedell.roke:matrix.org`, rather than any of those, when they attempt to.

#### How would you like to achieve it?

I envision this operating similarly to a room replacement, and/or upgrade.

Due to what [`mastodon/mastodon/issues/15320`](https://github.com/mastodon/mastodon/issues/15320#issue-762556521) describes also being yet unresolved, Mastodon already provides a solution:

- [`mastodon/mastodon/issues/177#event-2648084970`](https://github.com/mastodon/mastodon/issues/177#event-2648084970)

- [`mastodon/mastodon/issues/21413#issuecomment-1324946238`](https://github.com/mastodon/mastodon/issues/21413#issuecomment-1324946238)

Something, that operates as that does, is all that I request:

### Have you considered any alternatives?

- An auto-response bot might serve as a fall-back, for the few technically capable of implementing one for their account. However, this is not available for most servers. Additionally, it still creates conversations on the old accounts, thereby wasting everyone's time.

- If [`matrix-spec-proposals/pull/4440`](https://github.com/matrix-org/matrix-spec-proposals/pull/4440#issue-4151947721) is implemented, one should be able to direct prospective communicators to their replacement account via that. However, it may be trivial to miss.

### Additional context

If any content, owned by the account (messages, rooms, and/or spaces), were to be migrated during this process, this would require the pre-requisite resolution of more issues:

- [`issues/2774`](https://github.com/element-hq/element-meta/issues/2774#issue-2899976501)

- [`issues/893`](https://github.com/element-hq/element-meta/issues/893#issuecomment-1369645394)

…and [would be more fragile](https://github.com/mastodon/mastodon/issues/40389#issue-5359995242). Consequently, I do not propose any migration of data. That is best discussed in a separate issue, of which I imagine that one already exists.

Start by reviewing the linked Matrix spec proposals and related issues, especially matrix-spec-proposals/pull/1228, matrix-spec/issues/220, and matrix-spec-proposals/pull/4440. Define the verifiable account-replacement behavior and its boundaries without migrating messages, rooms, or spaces; the linked prerequisite issues show that data migration is out of scope.

Written by the indexing model from the issue text.