How do migrations work?

[r5h]

I've been reading the automerge doc section on migrations, and frankly it's a bit terrifying. As the (AFAICT) largest repo using of automerge, how do you guys handle changing the schema?

The simplest approach (with a single authoritative server serving both code and data) I can imagine is disallowing sync after a code update until the client has migrated it's local data to match the single sync server's version. Is something like that easy to do with automerge?