The ability to do a schema migration consisting of several changes as an all-or-nothing, all-at-once change would be useful even if they can't figure out how to

The ability to do a schema migration consisting of several changes as an all-or-nothing, all-at-once change would be useful even if they can't figure out how to do it concurrently with other operations on the tables. It shouldn't require too crazy of an implementation to allow reads concurrent with schema modification (seeing the old version of the world) and it's not impossible to allow concurrent, transactional writes too.