I don't really know how this worked, but I'll try 10man to 25man next shutdown.
In short,
I was soloing Ulduar 25man, cleared a few bosses before I noticed server+instance shutdown was soon (only 15minute restart). So I sat and exited the game whilst still inside the instance. Servers came back up and I was still inside Ulduar so I continued, wiped on Mimiron and ran back in. Didn't notice until I'd got Shadowdodger (10 player) that the raid had been switched to 10man. Walked out, changed back to 25man and finished Yogg.
Here's a screenshot: https://i.imgur.com/6GsnjWo.jpg
note lockout says 25 player.
So maybe when I died, and got ported outside the instance for the first time since restart, it auto-changed to 10man?
Main advantage of this 'bug' would be getting missing achievements without wasting a lockout, or 25man achievements in 10man, seeing if the difficulty setting changes too? Maybe get a mythic achievement in a heroic lockout?
Can't confirm just yet, just wanted to share incase another realm is getting a quick restart.