When a project has a need for a schema migration in regards to a NoSQL database makes me think that you are still thinking in a Relational database manner, but using a NoSQL database.
If anybody is going to start working with NoSQL databases, you need to realize that most of the 'rules' for a RDBMS (i.e. MySQL) need to go out the window too. Things like strict schemas, normalization, using many relationships between objects. NoSQL exists to solve problems that don't need all the extra 'features' provided by a RDBMS.
I would urge you to write your code in a manner that doesn't expect or need a hard schema for your NoSQL database - you should support an old schema and convert a document record on the fly when you access if if you really want more schema fields on that record.
Please keep in mind that NoSQL storage works best when you think and design differently compared to when using a RDBMS
