Nosql doesn't solve the schema migration problem. It just means you don't formalize your schema. But your code will implicitly require a certain schema anyway. Changing the schema means changing the code and migrating data. You'll have to write migration scripts and think about backward compatibility.
Same problems as in sql.
The trick is maintaining a full graph of all data dependencies through the entire codebase. Then migrations can be done with ease. But no one does this. They shovel data from one database to the next, with tons of little adhoc data stores along the way.
