Comment by 0xbadcafebee

Database changes are typically one-way. If your new change includes creating or modifying a table, such that there are new additional columns, and you populate those with data, then downgrading would destroy the changed columns and the data in them. Hence you can't downgrade once you upgrade or you'd potentially be breaking things. To downgrade safely you'd need to backup or snapshot the old database, and then restore your database back to the backup/snapshot, but that's not blue/green.