лучшая практика направляющих - фиксирует с подрывной деятельностью VCS

You shouldn't have to delete any migrations. All migrations should be reversible. If you are 100% sure that no one else has used those migrations, you can delete them by:

svn rm db/migrate/your_migration
svn commit db/migrate

However, if what you mean by rolling back, is fixing it for those who already used that migration, the way to go is creating a new migration that reverses the troubled one.