The migration comment has already been addressed.

"Denormalization is not somthing you have to deal with in MongoDB."

What do you mean exactly? If you have a piece of data, lets say an address, that exists in more than one place as is the mongodb way, you will absolutely have to deal with the fact that you need to now update that data multiple times. So denormalization is very mcuh some you have to deal with in Mongodb.

Just like in a relational database, denormalization has serious consequences and should only be used when the need for performance outweighs the risks of having much more complex code and hence potentially many more hard-to-debug errors.