Some ORMs do a decent job generating a correct sequence of DB queries for quite complex schema changes, including relationship changes. However, the process is

Some ORMs do a decent job generating a correct sequence of DB queries for quite complex schema changes, including relationship changes. However, the process is a bit more manual than you might think. For example, to change from 1-n to n-n you would first generate n-n for first migration, then a manual migration to move data, and then for the final migration you would remove 1-n. The first and last one can be auto-generated by making changes to entities, including the reverse migration. Of course, manual review is absolutely necessary and we write queries ourselves if they involve moving the data.