Solve upgrading private rooms before the big v12 upgrade round?

**Description:**

I recently upgraded a room from v5 to v10 and thought it was a client issue with fractal, so I opened an issue:

https://gitlab.gnome.org/World/fractal/-/issues/1655

> ### when upgrading a room version automatically reinvite all users who had been in the old room.
>
> I upgraded a room with fractal 12.beta+14 running on openSUSE Tumbleweed. and afterwards I was alone in the room. the other users havent been reinvited. this could become especially useful with the global bump to v12 rooms on 2025-08-11.

But the reply seems to indicate this is a generic issue.
I wonder if this can/should be solved before the big v12 upgrade round coming.

Start with the reported behavior during the v5-to-v10 room upgrade and compare it with the linked Fractal issue 1655, then determine whether Synapse is responsible for reinviting former members. Done means establishing the generic room-upgrade behavior and implementing or documenting a resolution before the v12 upgrade round.

Written by the indexing model from the issue text.