What is the difference between
rename_column :posts, :title, :subject
rename_column :posts, :subject, :title
rename_column is used in both
down because there's no need to change any of the column details other than the name. Both
subject are relatively short, text-based values. If you dropped a column and added a new one, you'd just be using the same datatype and other attributes. Renaming a column is simply more expedient in this case.
Just because you're changing a column doesn't imply that you have to change everything about it.