How do migrations work?

I've been reading the [automerge doc section](https://automerge.org/docs/cookbook/modeling-data/#versioning) 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?