Improve UX of Upgrading Rooms

### Your use case

#### What would you like to do?
Provide a way for rooms to be upgraded in place rather than split off a new room.

#### Why would you like to do it?
Because the current UX of having a new room split off from the current room is terrible, often times the 'old' room cannot be rejoined especially if it's a private room even though the user had permission before. Also it is very confusing, frustrating and upsetting for the user to lose access to old messages and to either be put into a new room or have to join a new room. Users coming from other messengers where this is either not a feature or handled seamlessly may write it off because of this and I wish users to have a seamless experience and to stay.

#### How would you like to achieve it?
Have the room upgrade 'in place' bringing all messages in line with the new room version rather than splitting off a new room or have the 'new' room contain all the old messages (not just a sign saying please access the old room) and for the new room to always be joined automatically by any user currently in or looking for the 'old' room.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

No files, tests, or entry points are named in this issue. Start by mapping the current room-upgrade flow across the relevant Element clients and room service, then clarify whether the goal is an in-place upgrade or migration with history and automatic joining. Done requires an agreed product and protocol design before implementation can be scoped.

Written by the indexing model from the issue text.