If your data are sufficiently big, you will probably find that you cannot EVER migrate the data, or that it is not beneficial to do so. This means that when you do a schema change, the code needs to continue to be backwards compatible with the old formats forever.
Of course if your data "age" and eventually expire anyway, this can do schema migration for you - simply change the format for newly added data, then wait for all data in the old format to expire - you can then retire the backward-compatibility code.
