@ericln to be exact, a schema change is indeed a bit different for noSQL schema, but really ment data manipulations like: create a new column and then populate this column with data from column a and b. This would'nt be really in the context of a migration anymore.
everything not structure related is some kind of data transformation and will be handled as "seeder" within db-migrate in the future. a migration only contains the structure change normally not any data operations that may exist. I worked already in many projects where the usage and abuse of migrations to use them to do such data manipulations just turned out to be very unstructured. There are times when you want to execute structure changes without applying the data manipulations and there are times you want both. And mixing those two things together ends up in a mess.
I'm currently working on the docs to all those things, you may have a look at them when I push them (I can notify you if you want), that should give a better idea of the whole thing.