MSC3818: Copy room type on upgrade (#3818)

* add proposal xxxx-copy-room-type-on-upgrade.md

Signed-off-by: Aminda Suomalainen <suomalainen+git@mikaela.info>

* MSC 3818: add MSC number

Signed-off-by: Aminda Suomalainen <suomalainen+git@mikaela.info>

* msc3818: remove template text from beginning

Signed-off-by: Aminda Suomalainen <suomalainen+git@mikaela.info>

* 3818-copy-room-type-on-upgrade: refer to the current version of spec and what to change

Signed-off-by: Aminda Suomalainen <suomalainen+git@mikaela.info>

* Update proposals/3818-copy-room-type-on-upgrade.md

Signed-off-by: Aminda Suomalainen <suomalainen+git@mikaela.info>

Co-authored-by: Andy Balaam <mail@artificialworlds.net>

* Update proposals/3818-copy-room-type-on-upgrade.md

Signed-off-by: Aminda Suomalainen <suomalainen+git@mikaela.info>

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

* 3818-copy-room-type-on-upgrade.md: specify v1.2 in links

Signed-off-by: Aminda Suomalainen <suomalainen+git@mikaela.info>

* 3818-copy-room-type-on-upgrade.md: correct link for MSC3588

Signed-off-by: Aminda Suomalainen <suomalainen+git@mikaela.info>

* MSC3818: note potential issue when state events are needed

* MSC3818: note an alternative of all room types needing an MSC

Co-authored-by: Andy Balaam <mail@artificialworlds.net>
Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

proposals/3818-copy-room-type-on-upgrade.md

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+# MSC3818: Copy room type on upgrade
+
+Unless the room upgrade API specifies that room type must be copied over, clients cannot rely on
+rooms staying the same type leading to trouble.
+
+
+## Proposal
+
+This MSC proposes that the room upgade API MUST copy the [room type](https://spec.matrix.org/v1.2/client-server-api/#types)
+over to the new room. Otherwise clients cannot trust that to happen and [Spaces](https://spec.matrix.org/v1.2/client-server-api/#spaces)
+or [MSC3588](https://github.com/matrix-org/matrix-spec-proposals/pull/3588) Story rooms may incorrectly become
+normal rooms breaking user-experience.
+
+The Spec currently specfies this in [section 11.32.3. server behaviour](https://spec.matrix.org/v1.2/client-server-api/#server-behaviour-16):
+
+> 2. Creates a replacement room with a `m.room.create` event containing a `predecessor` field and the applicable `room_version`.
+
+It becomes:
+
+> 2. Creates a replacement room with a `m.room.create` event containing a `predecessor` field, a
+> `type` field set to what it was in the previous room (if it was set), and the applicable `room_version`.
+
+
+## Potential issues
+
+Some room types such as Spaces also require copying over state events as a part of the update progress,
+in case of Spaces, `m.space.child` events. However as that can be changed later and done by the client,
+it's out of scope for this MSC.
+
+## Alternatives
+
+A suggested alternative is having every room type specify their own update process if they use other
+room types. However this would complicate the MSC process with even simple client-side proposals
+requiring also a server-side implementation. This could also result in room types dependent on a
+particular server software or discourage using Matrix for a smaller project where an MSC wasn't
+otherwise consider necessary.
+
+## Security considerations
+
+Non-applicable.
+
+## Unstable prefix
+
+Non-applicable.
+
+## Dependencies
+
+Non-applicable.