Migration is hard, but I fail to see how something like MongoDB makes this easier.
Either you kick the can to the code, or do a migration script. Is there some magic style of coding that allows me to use the old data structures with the new code without having to do any special checks?
What about removing a column? What about if you've decided that you both current and next version of the code to work (then removing a column is a 3 step process, copy to new and use new, skip a version, remove old).
Migration can and is often hard.