I don't fully understand this task. What do you mean with "rooms that are still on :matrix.org?" Matrix rooms aren't "on a server", they are shared by all rooms participating in it and there isn't a central authority.

The only thing I would make sure is that the main alias of every KDE room ends with :kde.org rather than :matrix.org.

@PureTryOut Rooms have a homeserver as far as I was aware and stuff goes through that? Earlier on we had rooms "moved" to our homeserver. I admit I don't have full knowledge of all the ins and outs of matrix protocol so I may be misunderstanding the implications of rooms havning for instance Internal room ID: !OoKhjuWNfhqSUbncIz:matrix.org or !qqzZaSFQlMmcwIFkly:kde.org I thought that was what that referred to?

Rooms have a homeserver as far as I was aware and stuff goes through that?

No. The internal ID you referenced really only indicates on what homeserver the room was created. But once more homeservers join in on the room (read: people from other homeservers joining the room) the room gets shared over all participating homeservers.

I doubt you had rooms "moved" to your homeserver, I think there were just some :kde.org aliases added and the rooms were switched from the matrix.org Freenode bridge to the KDE Freenode bridge.

we spell ourselves freenode, lowercase f, and in general we would want that bridge to be used, not the matrix one. (due to both limits and lag)

So I think it's

1. the aliases
2. the IRC bridge

that need to be changed where needed.
But I'm not terribly into Matrix internals, just IRC internals.