I just played Roving Clans for the first time. Next to my main city I build two districts. Then I decided to move my city closer to two anomalies, and packed it up. After moving the city to the new hex, I unpacked it again, but all districts where "lost". How can I move districts? Or will districts stay in the hex they were initially built on, and will be "reattached" to the city when they are part of the city radius again, but either moving the city again, or connecting to the district hex via other districts?

After you replant a city, you can replant its districts using Happy Decampers in the city's build menu.