Shut down local server

We moved to the cloud with Sophos Mobile Control. (Sophos Central Mobile) 
We migrated all devices. After we shut down the old server, the synchronization no longer worked correctly. 
The migration checklist states that you have to leave the old server running because the devices still communicate via the old server. 

https://doc.sophos.com/support/migration/de-de/mobile/MigrationChecklist/index.html 

Does this mean that the old server has to remain on permanently despite the migration?