DB schema generated by a migration may look different in rails 4 and 5 (because rails 5 may use different default values). For this reason it's important to explicitly set for which rails version a migration was written for. See https://stackoverflow.com/questions/35929869/activerecordmigration-deprecation-warning-asks-for-rails-version-but-im-no/35930912#35930912
Migrations shouldn't fail RuboCop checks - especially lint checks, such as the nested method check. To avoid changing code in existing migrations, add the magic comment to the top of each of them to skip that file.